How appropriate that as interest stagnates in Zynga’s hit Facebook game Farmville, players are now moving into the city with Cityville.

Just 18 days after launch, Cityville has 47.9 million monthly active users, Inside Social Games reports. At this pace, Cityville could soon top Farmville, currently with 56.3 million users, as Zynga’s most popular game and the biggest game on Facebook. It’s already Facebook’s fastest-growing game ever.

Inside Social Games guesses at four reasons for Cityville’s growth: aggressive cross-promotion in other Zynga games, localization for countries outside the United States, a higher level of polish than previous games and incentives for players to tell their friends about the game (despite Facebook’s policy against this practice).

Whatever the reason, I’m impressed that Zynga has created another hit that’s approaching the scale of Farmville. The only other Facebook games to come close are Frontierville and Texas Hold ‘Em Poker — both by Zynga — with 37 million and 38.5 million monthly active users at their respective peaks.

But as with all of Zynga’s big hits, interest wanes over time, and the number of people actively playing games like Farmville and Mafia Wars never returns to peak levels. Farmville, for example, once had 83.8 million monthly active users. I bet the same thing will happen with Cityville.

The only recourse for Zynga is to introduce new games, and I’m not convinced this strategy will work forever. In addition to the reasons for Cityville’s growth mentioned above, the game could be a hit because the concept is dead-simple, like Farmville, but modern. How many more of those winning ideas can Zynga come up with? We’ll have to wait for the inevitable Suburbville or Spaceville to find out.

 
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