Elgg is free and open source social networking platform that powers all kinds of social environments, from a campus wide social network for your university, school or college or an internal collaborative platform for your organization through to a brand-building communications tool for your company and its clients.

If you are looking to create your own social application, want to build and run a site for your organization, or introduce a social layer into your technology stack, Elgg is a great choice.
Elgg 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.