BuddyPress is a suite of WordPress plugins and themes which extends WordPress MU and brings social networking features to a new or existing installation.
The Social Media Classroom is a new project started by Howard Rheingold which offers an open-source Drupal-based web service to teachers and students for the purpose of introducing social media into the classroom. The service includes tools like forums, blogs, wikis, chat, social bookmarking, RSS, microblogging, widgets, video conferencing, and more.