The game launched on December 2, 2010 and grew faster than any game in history. CityVille now has 61.67 million monthly active users, compared to 56.85 million for FarmVille. That number will dip a little at some point: all games, social or not, aren't played by everyone who tries them out. Right now though, CityVille is still growing, and we don't know where its number will eventually settle.
Zynga has something like 261 million monthly active users across its various social games. Like all of Zynga's games, CityVille is free, but users can spend real money on virtual goods to enhance their experience of playing the game. Zynga makes a killing in revenue every year from the sale of virtual goods. Two months ago, the company was valued at $5.51 billion, which is more than Electronic Arts.