BuddyPress - WordPress social networking in a box.
SocialEngine - Socialengine PHP: Elevating Your Online Community Experience.
Elgg - Opensource social network engine
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.
phpFox - phpFox is a powerful social network platform for niche communities
HumHub - HumHub is a free social network software and framework built to give you the tools to make teamwork easy and successful.