update : 23/12/2020 Drupal Commons

Drupal Commons is a Drupal oriented social networks distribution published by Acquia (a services company founded by Drupal's creator) in 2010. Drupal was designed in the 2000s by Dries Buytaert and has enjoyed worldwide success since then.

The scope of the community portals that can be created using Drupal depends on the available modules (more than 10,000 – although not all of them concern CSNs). Some relevant examples include Homebox (for personalised pages), Invite Friends (friend management), Content Profile (profile data management), FriendList (list of friends), Organic Group (groups management), Privatemsg (private messaging), and more. The Drupal Commons distribution has a broad native scope capable of rapidly implementing a real social network. Version 3.0 also introduces responsive design, thus offering compatibility with mobile devices.
Drupal is published under the GNU GPL and is 100% open source. A support option is available from Acquia. In fact, it was the full package configuration, Drupal Commons, which we examined here.
Technically speaking, Drupal is built around LAMP technologies (PHP and MySQL in particular). The latest version, v7, will offer greater stability and a broader functional scope.