Facebook has released guidelines for social design, in order to promote “community, conversation and identity” when building applications on the Facebook platform. These recommendations include automatically connecting existing Facebook friends, display social context and connections whenever possible, and encourage interaction though conversation starters.
Largely, the guidelines are based on the premise that the Facebook community already exists. You’re building a community within a community. So, your strategy should be to utilize the existing community that exists, and define a new conversation that users can build around.
Facebook is committed to evolving their Social Design Guidelines over time, and publish how-tos to help Facebook app developers improve their community building via Facebook.