Connect. Share. Collaborate.

Elgg is free and open source software
It provides the components you need to build a fully-featured social environment, including:

Profiles
Elgg comes with default profile fields that can be extended or completely customized by the site administrator.
Activity Streams
The granular activity stream feature ensures you keep up to date with all site activity.
Blogging / Microblogging
Blogging and microblogging functionality that is both powerful, yet simple to use.

Groups
Collaborate around specific topics via public, private or hidden groups.
Access controls
Control who can access your content.
Web Services API
Expose functionality through the REST API by building a plugin and then either publish the API for other developers to build clients or provide your own.