Elgg - Opensource social network engine
SocialEngine - Socialengine PHP: Elevating Your Online Community Experience.
phpFox - phpFox is a powerful social network platform for niche communities
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.
mooSocial - mooSocial is the best social network script to create a niche community or social site.
Zoho Social - Social media management software for growing businesses and agencies. Manage multiple social networks, schedule unlimited posts, monitor keywords, and collaborate with your team — all from a single dashboard.