Traffic stayed level or fell for most of the biggest Facebook games during December, according to AppData. 10 of the top 25 games saw decreases from December 1, 2009 to January 1, 2010. Another 5 games saw gains of less than 1 million.

We’ve previously seen drops in traffic on this monthly leaderboard. Over the course of November, 8 of the top 25 games had seen traffic drops — but mostly drops of less than 1 million users, whereas this past month almost every game that fell did so by more than 1 million. The reasons for these drops are likely a combination of perennial holiday season decreases together with Facebook’s stricter enforcement policies against some viral growth tactics.

However, the past month also saw major gains for some apps and their developers. Here’s a closer look.

• Zynga had a mixed month. Falling titles included long-time favorite Mafia Wars, YoVille, and Roller Coaster Kingdom. However, its successful launch of PetVille in early December brought in nearly 18 million users. The continuing popularity of FarmVille — still by far the most popular application on Facebook — and FishVille have helped buoy the numbers.
• Playfish‘s biggest apps continued to drop, even as newer titles, like Poker Rivals, slowly make their way up the leaderboards. The company has no doubt been busy integrating with its new owner, Electronic Arts, so we expect it to make a big comeback in the next few months.
• RockYou shows up on our leaderboard this month, for a rather strange reason. It launched Zoo World this past fall, and integrated a window of the game into its other applications, notably Birthday Cards. Meanwhile, it has designated Birthday Cards as a game in Facebook’s application directory, which means it shows up that way in AppData. The kludge integration seems to be working, in any case, as Zoo World and Birthday Cards have seen considerable growth in the past month.
• CrowdStar has been on the rise all fall, starting with the launch of Happy Aquarium. While that game appears to have leveled off in popularity, newer titles including Happy Pets and Happy Island are coming up.