BuddyPress - WordPress social networking in a box.
Bitrix24 - Bitrix24 is a free (for small businesses) social enterprise platform. It is a united work space which handles the many aspects of daily operations and tasks. Create your own social intranet in minutes!
SocialEngine - Socialengine PHP: Elevating Your Online Community Experience.
LinkedIn Premium - LinkedIn is a social network helping brands and professionals to connect, identify sales prospects, hire talent, and promote their business.
Elgg - Opensource social network engine
Salesforce Chatter - Salesforce Chatter provides a suite of collaboration features to any application built on Force.com.