You can use www.elgg.org a powerful open source social networking engine
Providing you with the core components you need
to build out socially aware applications
There is also http://www.pligg.com/
Pligg is an open source CMS (Content Management System) that you can download and use for free. Pligg CMS provides social publishing software that encourages visitors to register on your website so that they can submit content and connect with other users. Our software creates websites where stories are created and voted on by members, not website editors. Use Pligg content management system to start your own social publishing community in minutes.