[Every other week, Gamasutra examines the most popular Facebook games according to monthly active users, looking at the top titles and developers on the social network to see who's attracting or losing players.] There's a bit of movement in our latest chart for Facebook's top 20 games, though the titles remain the same for the most part -- the only game to slip out of the list since our last tally two weeks ago is Zynga's Pioneer Trailer, which went from #20 to #21, as the company's CastleVille elbowed its way in. Zynga's CityVille continues at the top of the tally, despite losing nearly 2 million users and coming down to 49.2 monthly active players. The company's FarmVille managed to gain around 300,000 virtual farmers and a spot as the game reclaims #2, now with 31 million users. The Sims Social by Electronic Arts/Playfish made way for FarmVille, coming down to #3 and losing 3 million players in the process. The game now has 29.8 million users. Zynga's Texas HoldEm Poker remains at #4, albeit while suffering a 900,00 setback that puts its user count at 29.8 million. Launched last month by Zynga, CastleVille, like all recent titles from the developer, has quickly shot up the app leaderboard list to #5. The medieval-themed social game now has 26.5 million players -- more than 9.1 million users tried CastleVille out in just the last week. Two more Zynga titles follow: Empires & Allies at #6 (17.8 million users, down 1.2 million) and Words With Friends at #7 (12.5 million users, up 800,000). After that is Wooga's Diamond Dash (11.4 million users, up 400,000) and Bubble Island (9.8 million users, up 100,000). The Zynga title that's suffered the most losses in the past two weeks is Mafia Wars 2, the company's first numbered sequel. It dropped from #6 to #10 after losing 6.3 million players. It now has with 9.7 million users. You can see the full list of the top 20 Facebook games along with exact monthly active user counts below: All charts and data are courtesy of Facebook application metrics and trends site AppData.