Elgg - Open Source Social Networking Engine
Elgg is an opensource PHP application which is great for Social Networking portals. Since it is social netowoking that's why elgg has introduced EAV database model to allow new features without change in any database structure.
It is a pluggable machine where you can create your plugins as per your requirement. I've never seen such a flexible framework where you can make changes up to any extent. It supports on Apache/Nginx, PHP and MySQL – the same open source platform that the majority of web applications are written in. Elgg is compatible with enterprise technologies like the Zend Platform and any server environment that can run the Apache web server. It has also been awarded as best open source social networking platform in 2008.
We are planning to create some serious plugins which will be released opensource in elgg community.