Watch out CityVille, there's another contender coming for your title as top Facebook game! Luckily, this new kid on the block is in the same staple, as Zynga can lay claim to both CityVille and rising star CastleVille. According to the latest stats on AppData, CastleVille has rocketed to 37 million monthly active players, passing FarmVille with its almost 34 million players to become the second most played game (currently) on Facebook.

But wait - where's EA / Playfish's The The Sims Social? The game still sits as the number five game on the service, but it has lost players (around 1.7 million) even that Playfish should try to help the game climb back up the ranks. And that was just a week ago!

Either way, does CastleVille have what it takes to finally topple CityVille as the number one game on Facebook? It still has 12.5 million more players to go (and that's if CityVille does nothing but lose between now and then). Still, it will be interesting to watch as the competition develops further (or doesn't) between these two games.