Yonkly Open Source twitter-clone and microblogging platform developed on the asp.net mvc framework.
demo:
http://www.yonkly.net/timeline

Ruby on Rails Open Source Twitter Clone Script. This is a group chat social network application. You can start conversations and connect wih other users while discussing it. It is possible to make a conversation read-only for presenting a content too.

demo:
http://kienthucvn.net/blog/index.php

BuddyPress will extend WordPress MU and bring social networking features to a new or existing installation. BuddyPress is a suite of WordPress plugins and themes, each adding a distinct new feature. BuddyPress contains all the features you’d expect from WordPress but aims to let members socially interact. All BuddyPress plugins can be themed to match your own style, in just the same way as a WordPress blog. The BuddyPress plugins are bundled with a default theme to get you going out of the box. As all BuddyPress plugins are independant, they can be used in two different ways. All of the plugins could be used to create a complete social network from scratch, or you could use specific plugins to add desirable features to your existing blog network.

demo:
http://testbp.org/