[Editor Eric Caoili of Gamasutra sister site Worlds in Motion presents a biweekly roundup of the most popular Facebook games (by monthly active users), looking at the top titles and developers on the social network to see who's attracting or losing players.] As it has been for months now, Zynga's FarmVille is at the top of the heap with 75.5 million monthly active users, though that is down from the 83 million high the farming sim attracted just two months ago. Many of the most popular social games have lost millions of players since Facebook made changes to how it handles application notifications, limiting the viral growth many titles enjoyed. RockYou!'s Birthday Cards isn't really a game, but is categorized as one anyway. The developer -- recently a recipient of the San Francisco Business Times' Technology and Innovation Award in the Social Media category -- has at least one real game on the top 20 list, though: Zoo World, which holds the #11 spot with 15.2 million users. Zynga games take up the #3 to #7 spots with Texas HoldEm Poker (28.3M users), Treasure Isle (27.7M users), Cafe World (25.2M users), Mafia Wars (22.9M user), and PetVille (19.3M users). CrowdStar's Happy Aquarium ($17.4M users) and Electronic Arts/Playfish's Pet Society (16.9M users) take #8 and #9 respectively before Zynga comes in again at #10 with FishVille (16.6M users). Outside of the Zynga-dominated top ten, you see a lot more variety in terms of developers (and game types somewhat). There's the aforementioned Zoo World, EA's Hotel City and Restaurant City, the MindJolt Games portal, Playdom's Social City, Zynga's YoVille, PopCap Games' Bejeweled Blitz, CrowdStar's Happy Island, Country Life, and Slashkey's Farm Town. All charts and data are courtesy of Facebook application metrics and trends site AppData.