Elgg is an open, flexible social networking engine, designed to run at the heart of any socially-aware application. Building on Elgg is easy, and because the engine handles common web application and social functionality for you, you can concentrate on developing your idea.

It runs on Apache, 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.

Elgg supports technologies like OpenDD, OpenID and OpenSocial, and they are directly involved in community efforts to push the envelope when it comes to data portability, federation and the user experience. Elgg is a great way to future-proof your social applications.

You can also look at other Open Source Social Network Platform we have mentioned few months ago. I am sure you can find the one which is suitable for you to start your own social network site.

Requirements: PHP 5.2+, MySQL 5+, Apache Server with GD and Mod_rewrite
Demo: http://elgg.org
License: GPL License