In designing communities, we found it worked best to take a small team and add complete functionality of certain social media functions all at once. For instance, one team member worked on adding twitter integration to the community, another worked on making sure that "friend of friend" functionality worked, another would concentrate on configuring the creation of community specific profile questionairres.

My suggestion would be to try to take your queues from the user community. Pick the features that are most needed. Don't bloat the website trying to become the next Facebook. There's plenty of web content around A-B testing so I won't get into that here.