BuddyPress is a WordPress MU powered social network platform being developed. BuddyPress will be more of a social networking around the member profile rather than just the blogging platform. However, members can still blog and use all the blogging features they would normally expect from WordPress.
BuddyPress has a set of WordPress MU specific plugins which would turn a standard vanilla installation of WordPress MU into a social network. BuddyPress has a core plugin that all other plugins require, it contains shared functions and performs the basic modifications to the WordPress MU interface. BuddyPress administrators can pick and choose the components for their installation, additional components could be written and other existing WordPress plugins could be used alongside.
The standard features of BuddyPress are Extended Profiles, Personal Blog, Private Messaging, Friends, Groups, The Wire, Status Updates, Albums. Visit BuddyPress and read more.