BuddyPress Social Network
From the plugin developers:
Are you looking for modern, robust, and sophisticated social network software? BuddyPress is a suite of components that are common to a typical social network, and allows for great add-on features through WordPress’s extensive plugin system.
Aimed at site builders & developers, BuddyPress is focused on ease of integration, ease of use, and extensibility. It is deliberately powerful yet unbelievably simple social network software, built by contributors to WordPress.
Members can register on your site to create user profiles, have private conversations, make social connections, create and interact in groups, and much more. Truly a social network in a box, BuddyPress helps you build a home for your company, school, sports team, or other niche community.