Did you guess - FarmVille?
As reported by the blog Econsultancy:
As the name suggests, FarmVille is a game that gives users the ability to "grow delicious fruits and vegetables and raise adorable animals" on their own virtual farms. With nearly 35m active monthly users and 12m active daily users, FarmVille has just about grown itself into the most popular Facebook application ever. When FarmVille surpasses 35,554,755 active monthly users, it will surpass the record set by the How Well Do You Know Me? application.
The author even goes on to (correctly, in my eyes) speculate that this is an app that would make Jim Rogers smile.