This year has brought a few surprises on Facebook. Playdom’s Social City, for instance, quickly grew to 12 million players with the concept of city-building, and EA’s Hotel City has done the same with virtual hotels. But the biggest surprise may have been the lack of blockbuster hits — the Happy Aquariums and Mafia Wars that took over Facebook last year, quickly snapping up tens of millions of players.

But Zynga has eventually pulled through. On April 2nd, it released Treasure Isle, a simple treasure-hunting game. It took Isle less than four weeks to pass Mafia Wars, one of Zynga’s biggest hits: last night Isle registered 24,315,796 players, giving treasure hunters a slight edge over the Mafia.

We’ve been tracking Isle since its release, so this news won’t be a huge shock to regular readers. Within five days of its launch, Isle had over three million players, and it has added over a million players most days since, consistently placing it at the top  of our AppData lists.

Isle also appears to have single-handedly reversed a decline in Zynga’s traffic that came as Mafia Wars and several of its siblings shrank slightly, giving the company a net gain of about 13 million monthly active users over the past month:

Treasure Isle is now Zynga’s fourth-largest game, although there’s no telling how much further it will go. Traffic to rapidly-growing apps can often level off suddenly if the publisher decides each new user is requiring too many advertising dollars. There’s also an initial sign that Isle is slowing; the same daily gain that saw it top Mafia Wars was also smaller than on all other days since the game first took off.

And even with the example of Treasure Isle to go by, it may not be as easy this year for developers to achieve huge numbers on new games as it was last year. Zynga’s other new game, Poker Blitz, hasn’t gone anywhere fast, and even the admirable growth numbers posted by Social City and Hotel City are low compared to the growth created by last year’s farming, pet and fish-raising fads.

Out of the three ways that games have typically grown, only two remain strong — cross-promotion (which Zynga can do better than anyone, because of its massive size), and spending (which Zynga can do because it has a lot of money in the bank). The third, viral growth, has been severely curtailed. One reason is that Facebook removed third-party notifications in March. Another is that it has been redesigning the user home page and other features in ways that improve the overall user experience but make it more difficult for games to reach users.