BuddyPress is a plugin for adding social network on top of wordpress cms.

The basic installation contains: activity streams, user profiles, avatar uploads, social groups creation, wire(status update and treaded discussion with @mentions), discussion forum in groups(provided by the integration of bbpress forum software), friend system, private messaging, blogs(via wordpress), traditional forum system(via bbpress). This list of functionalities can be extended by numerous third party plugins and themes.