Elgg is a open source social networking engine, that can help you build your own social site, public or private.
Elgg is an open source social networking engine, upon which a web based social network or application can be built.
Elgg is written in PHP, and also uses JavaScript. MySQL is used as the database backend. Elgg is open source, released under the GNU General Public License v2 and the MIT License.
The central location for discussion about Elgg is http://community.elgg.org/. Developer documentation is here. Google Groups also has an Elgg development and an Elgg users group.
More information: elgg.org
A commercial turnkey/hosting option is also available through elgg.com.