Like Spotify, MOG lets you listen to any song or album you want from a library of about 11 million tracks. But unlike Spotify, MOG’s free service isn’t strictly time-limited. (Spotify users get six months of unrestricted listening, followed by 10 hours per month and five plays per track.) Instead, MOG uses a game-like system that rewards certain actions with more free listening. Refer some friends, get some free time. Recommend a playlist, get more free time. Click on an ad, get more free time.

MOG won’t say exactly how much free listening you’ve got or how much you’re earning. Free listening is represented by an ambiguous meter that works like a gas tank (that is, before cars told you how many miles you had left). But MOG says that if you’re got a lot of clout through social media, you could theoretically get free music forever.

The whole system is the service’s way of trying to go viral. To that end, MOG is launching a new HTML5 app that’s easier to use than MOG’s old website and a lot more convenient to access than Spotify’s installed application. David Hyman, MOG’s founder and CEO, coyly told me that if Facebook were to “hypothetically” launch a music service of its own (as rumored and expected), MOG would allow users to log in through the social network.

In a couple of months, MOG will start showing ads on its website, but what the company really wants is for people to eventually sign up for its paid service, which for $10 per month includes smartphone access and unlimited listening. The premium version of MOG is good enough to replace iTunes and downloadable music entirely, but these  kinds of services have never gone mainstream because people don’t like the idea of leasing songs instead of owning them. That’s a problem, because they’re unsustainable on ads alone.

Ideally, free versions MOG and Spotify will convince people that unlimited streaming music has its virtues and that paying for the service is worthwhile. But they also risk reinforcing the idea that music should be free, prompting record labels to pull the plug on licensing. I hope the former scenario is the one that pans out, because I like these services and want them to stick around for a while.

Now, TechCrunch reports that a Turntable.fm iPhone app is coming soon, and the site has a handful of screenshots to prove it. (Co-founder Billy Chasen seems to have confirmed the rumor, writing in the comments that “We were saving this as a surprise for [TechCrunch's Disrupt conference] when I’m on stage.”)

That’s great news, but it also makes me wonder whether the free ride on this very cool music service is coming to an end.

At the moment, Turntable.fm costs nothing to use. Its entire library of millions of songs can be listened to for free, with no time limits or restrictions on skipping tracks, as long as one other user is taking turns being the DJ. And I’ve never seen a single advertisement on the website.

That’s not to say Turntable.fm isn’t paying to deliver music to users. Although Turntable.fm argues that it doesn’t have to make deals with record labels, it still has to pay music composers for performance rights, and has already signed deals with ASCAP and BMI. There’s also no guarantee that record labels won’t start arguing for direct licensing deals. The legality of the service depends on Turntable’s interpretation of the Digital Millenium Copyright Act, as All Things Digital’s Peter Kafka points out. And in any case, Turntable will eventually want to make money for itself.

The launch of an iPhone app seems like a good opportunity for Turntable.fm to start thinking about revenue. So I wonder: Will it require a subscription? Will we suddenly see restrictions on desktop listening, akin to Pandora’s 40-hour per month limit? And will Turntable.fm offer some kind of membership with other perks, like higher quality streams and exclusive avatars? I’m guessing many of these questions will come up next week. I’m interested to hear Turntable’s answers.

Naturally, that leaves a lot of unanswered questions. Here’s what I’d most like to know about Spotify’s U.S. launch:

When’s the launch date?

Spotify’s U.S. launch plans aren’t a big surprise, even if they’re now official. The bigger question is when the service will be available, and to how many people? All we know is that the service will be invite-only at first.

Does Spotify have all the deals it needs?

To launch in the United States, Spotify needed new deals with record labels, and it’s not yet clear whether all those deals are in place. The big four record labels are key, but equally important are the indie labels that appeal to more obscure tastes.

How much music will be free?

As Macworld’s Christopher Breen notes, Spotify’s current terms of service provide six free months of ad-supported music. After that, non-paying customers get 10 free hours per month, but may not listen to a single track more than five times. Before November 10, users in Europe didn’t have these limitations. Whether U.S. users will face even more restrictions is unknown. (The premium service costs 10 pounds per month in the United Kingdom, or 5 pounds without mobile access. Presumably that will translate to $10 and $5 per month in the United States, same as competing subscription music services.)

Will Facebook integration be part of the launch?

GigaOM’s Om Malik recently reported that Facebook is preparing to launch a music service that integrates Spotify and other sources. Facebook, meanwhile, is preparing to launch lots of new services in the coming weeks and months, starting with today’s video chat announcement. It sounds like Spotify and Facebook will be intertwined, but maybe not right away.

How will other music services respond?

Spotify’s entering a crowded field in the United States, one that includes MOG, Rdio, Rhapsody and Zune Pass. But none of these services have freemium offerings beyond a brief trial period. MOG has said that it plans to offer some music for free. Will other services follow suit and essentially blend Spotify into the pack?

Will the masses ever pay for music they don’t own?

From what I’ve observed, lack of ownership is still a huge stumbling block for subscription music services. Zune Pass is the only one that lets users keep songs after they’ve stopped subscribing, but it costs $5 more than its competitors for 10 free songs per month. As Spotify has put a greater emphasis on converting users to premium memberships, it’ll have to convince users that instant access to everything is more valuable than ownership of a limited library. So far, Spotify’s competitors haven’t sold the masses on that concept. Maybe the rumored Facebook integration will help Spotify do better.

Skype has a $6 million investment in Rdio, thanks to some lawsuit madness involving Skype’s founders and several Silicon Valley players. Kafka said he’s “pretty sure” Skype and Rdio were planning to deepen ties and drum up more users for the music service.

But Microsoft has its own music service, Zune Pass, and it seems unlikely that the company will want to manage a competitor. For now, neither Microsoft nor Rdio are commenting. (UPDATE: See the end of the post for Rdio’s statement.)

Although Zune Pass and Rdio both offer unlimited streaming music to subscribers, their approaches are quite different. Zune Pass focuses on Microsoft’s own hardware — Xbox 360, Zune media players and Windows Phone 7 — and it has one pricing model, charging $15 per month for unlimited streaming and 10 permanent downloads. Rdio charges $5 per month or $10 for mobile access, and doesn’t let users keep any songs, but it supports more devices, including iPhone, Android, Blackberry, Windows Phones, Roku and Sonos.

So the best case scenario, however unlikely, would be a complete merging of Rdio and Zune Pass, giving users the full range of pricing options and supported devices. The services could even combine their respective social networks to allow more interaction between users.

The worst case scenario would be the closure of Rdio. It wouldn’t be a total tragedy — the subscription music business has enough competition from MOG, Rhapsody, Grooveshark and maybe Spotify to soak up displaced users — but it would be an inconvenience for Rdio users who’ve built up huge libraries and networks of like-minded listeners.

Whatever the case, I hope Rdio (or Microsoft or Skype) clarify the matter before long.

UPDATE: Here’s a statement from Rdio:

“While Skype is an investor in Rdio, it is important to note that it is one of many and that Skype’s acquisition has no impact on Rdio, operationally or financially. We remain focused on our goal of continuing to deliver what we believe to be the best online and mobile music experience on the market.”

It’s an idea that takes getting used to, and most people aren’t willing to take the plunge for $10 per month. So I’m not surprised that MOG is working on a freemium business model that it hopes to introduce in a couple of months. Evolver.fm’s Eliot Van Buskirk reports that MOG will give away limited access to its streaming catalog in hopes of roping in more users.

Outside the United States, Spotify runs on a similar business model, giving away 10 hours of streaming music and putting smartphone access behind the paywall. But stateside, record labels are skittish about such a generous freemium model, fearing that it could cut into sales on iTunes and elsewhere. That could explain why Spotify has been cutting back on the amount of music it gives away as it moves toward a U.S. launch.

How MOG will limit free users is unclear, but Van Buskirk says video advertising — and more advertisements in general — could keep the free service from bleeding to death on licensing costs. The paid version, of course, is ad-free.

In any case, going freemium will be a gamble for MOG. I’ll bet MOG will lose some customers who think they can get by on the free version. But without the barrier of credit card entry, more people may get accustomed to streaming from a huge music library instead of owning a small one. If my anecdotal experience is correct, that’s a big hurdle to clear.

Unlike other streaming music services, such as Rdio and MOG, Grooveshark doesn’t license the entirety of its library. Songs are uploaded by other users, allowing Grooveshark to undercut the competiton with free web streaming and a $3 per month mobile app. Although Grooveshark has made an arrangement with EMI, a lawsuit against Universal Music Group is underway, and I wouldn’t be surprised if record labels complained. Apple yanked Grooveshark’s iPhone app last July.

But as CNet’s Greg Sandoval points out, the timing of the Android app’s removal is coincidental. Google’s general counsel, Kent Walker, is testifying on Wednesday before the House Judiciary committee, which is investigating websites that profit from piracy. Google is fending off allegations that it makes money from AdSense ads on piracy websites, and will defend its reputation by pointing to its own anti-piracy efforts.

For instance, in December, Google announced several new measures, including the removal of “BitTorrent” and other piracy terms from its auto-complete search results and an improved anti-piracy review for AdSense. It’s not clear whether the removal of Grooveshark was politically motivated, but it could help the company make a stronger case before Congress.

Just one other observation: Grooveshark’s Chrome web app remains untouched, at least at the time of this writing. Granted, the Chrome Web store is much lower-profile than Android, but I’ll be interested to see whether Google starts policing this ecosystem for copyright violations as well, even if its just a collection of glorified bookmarks.

Sure enough, a Google search for Lady Gaga — the requisite artist for all new tech ventures related to music — no longer serves up a box full of streaming songs. A special landing page for the service now re-directs to a general list of search features, with no mention of Google Music Search.

What happened? Google hasn’t said. In fact, this story could be quite old, given that the service vanished without a whisper. But over the last year and a half, the two main music services that powered Google Music Search, Lala and iLike, were acquired and subsequently killed by Apple and MySpace, respectively. R.J. Pittman, one of the main forces behind Google Music Search, now works for Apple. And a TechCrunch commenter who claims to have worked for iLike said adoption was minimal from the very beginning. With the driving forces out of the picture, I’m not surprised to see Google quietly put Music Search down.

Then again, I wouldn’t be surprised to see Music Search’s demise even if the driving forces were still in place. The amount of music available on YouTube — official and otherwise — makes a single-serve streaming service from Google redundant, especially when YouTube results get top priority in search results. And then there’s the rumored Google Music, a store and digital locker that’s supposedly near completion. If that’s true, Google Music Search was approaching obsolescence anyway.

If you’re still dying for streaming music to show up in search results, consider Yahoo and Bing. Both search engines have all the Gaga you could ever want.

MOG costs $10 per month for unlimited mobile access to its on-demand streaming music library, the same price as competitors Rdio, Napster and Rhapsody. After the music labels and publishers get their share, and after MOG pays other fees for things like bandwidth, hosting and reporting of listening data, the company will lose money on every in-app subscription if Apple takes a 30 percent cut.

“We don’t understand why Apple should get more from our business than we get,” MOG’s founder and chief executive David Hyman said in an interview.

MOG’s dilemma is similar to that of all subscription-based music services doing business in the iOS App Store. After Apple announced its subscription policy in mid-February, Rhapsody President John Irwin said the arrangement is “economically untenable.” He added that “we will be collaborating with our market peers in determining an appropriate legal and business response to this latest development.”

Hyman didn’t mention legal action to me, but he said higher prices are “certainly an option.” MOG may also try to convince record labels to pay a portion of Apple’s cut, but Hyman wouldn’t comment on whether those negotiations are happening now. When I asked if MOG would consider an exodus from iOS, Hyman said “I don’t know the answer.” But to illustrate how MOG doesn’t rely on iOS, he listed all the other platforms on which MOG is available, including Android phones and Roku set-top boxes. The company recently struck a deal with Verizon Wireless to get pre-loaded on Android handsets with optional carrier billing.

For now, Hyman is avoiding any decisions on iOS until he’s clear on how Apple’s rules apply. A supposed e-mail from Steve Jobs to an iOS developer said Apple created the in-app subscription policy for publishers, not software-as-a-service. Hyman’s not sure under which category MOG would fall.

“They cast a wide net and some people get caught,” Hyman said. “I think that’s why you’re starting to see revised statements from them.”

Apple has reportedly given publishers until June 30 to implement in-app subscriptions, or else they’ll be booted from the App Store. As Harry said, the next four months are going to get really interesting.

Speaking to The Age, Sony Computer Entertainment Australia Chief Executive Michal Ephraim said his company would like to get away from iTunes, if only it could move to a credible alternative, such as the Sony Music subscription service that’s rolling out now.

But thanks to some eye-catching headlines (including The Age’s own), Ephraim’s remarks got twisted into a threat to abandon the most popular music download service in the world. You need only look at the quotes to see that’s not the case.

Here’s what Ephraim said:

“If we do [get mass take up] then does Sony Music need to provide content to iTunes? Currently we do. We have to provide it to iTunes as that’s the format right now.”

In other words, Sony realizes that it’s powerless to leave iTunes for as long as digital downloads are the main way that people consume music.

Ephraim continues:

“Publishers are being held to ransom by Apple and they are looking for other delivery systems, and we are waiting to see what the next three to five years will hold.”

So not only does Sony understand that it needs subscription music to take off before it can even consider leaving iTunes, it also recognizes that the transition isn’t going to happen in the foreseeable future.

The idea that Sony would just pack up all its music and leave makes for an intriguing narrative; Apple just dissed Sony big time by rejecting Sony’s eReader application from the iOS App Store. But there’s no retaliation going on here. Sony simply hopes that new business models will shake up the digital music market so that it’s no longer dependent on Apple for survival. Isn’t that what every iTunes publisher wants?

One more thing: If Sony was seriously going to abandon iTunes, I think we’d be hearing about it from Sony Music Entertainment, the record label, not the head of the company’s gaming division in Australia.

Effective February 15, Last.fm will charge $3 per month for access on iPhones, Android phones and home entertainment devices such as Sonos and Logitech’s Squeezebox. The exceptions are Microsoft’s Xbox 360, which includes Last.fm with a $50 per year Xbox Live subscription, and Windows Phone 7, which will remain free through 2011. Ads will be removed as part of the shake-up.

Last.fm’s website will remain free with advertising in the United States, United Kingdom and Germany, but Matthew Hawn, Last.fm’s vice president of product, explained in a blog post that an ad-supported service is simply “not practical” on other devices and in other countries.

Even if Last.fm paints this change as a necessity, it’s clear that the service hasn’t withstood licensing and royalty demands as well as competing services.

Pandora, for example, offers a $36 per year premium service but still allows 40 hours of free streaming per month. After that, you can pay $1 to keep listening. Slacker Radio gives away its basic service and charges $5 per month for premium features. On-demand services such as MOG, Rdio and Rhapsody charge $10 per month for streaming that can effectively replace your music collection. Outside the United States, Spotify offers ad-supported, on-demand music but charges a subscription fee for smartphone access.

The real problem is that Last.fm doesn’t have a tangible advantage over these other services, aside from Hawn’s subjective claim that its version of Internet radio is “the best in the world.” Without a freemium business model, Last.fm is going to have a tough time showing smartphone users what it has to offer.