I can't say that I am a fan of any of the popular Facebook games but do like the idea of a game which aims to do some good. My Conservation Park by Good World Games, " is a socially conscious Facebook game that places you in charge of building and managing a protected wildlife reserve". Your goal is to help protect endangered animals and their habitats, while at the same time aiding with real life conservation efforts. The site states that they donate part of the game revenues to non-profit conservation groups but does not specify how much.
Tired of Farmville and want to help make a difference while gaming? This game may be worth checking out.