BuddyPress - WordPress social networking in a box.
SocialEngine - Socialengine PHP: Elevating Your Online Community Experience.
Elgg - Opensource social network engine
mooSocial - mooSocial is the best social network script to create a niche community or social site.
JomSocial - Open Source Social Networking Software. Award winning open source community software by JomSocial. This social networking software is widely used by many Joomla users.
HumHub - HumHub is a free social network software and framework built to give you the tools to make teamwork easy and successful.