Friendika is a distributed social network application which runs on commodity hosting platforms – PHP/MySQL/Apache. (If you can install Drupal or WordPress you can probably install Friendika).
A single Friendika installation supports up to several thousand members.
But here's the cool part… Every Friendika installation can connect with any other to create a social network without limits. (And one that nobody can own or control).
Friendika can also connect to your friends in other social networks – such as Status.Net, identi.ca, GNU-social, and Google Buzz. The list of supported networks is growing.