The best open source social networking script I’ve found by far is Elgg. Developed as sort of a “myspace for academics,” Elgg is a powerful social networking solution for any organization. Reasonably easy to install and customize, I’d recommend Elgg for any company or organization that wants to get up and running quickly and inexpensively.
When I say “reasonably easy to install” I mean, not as easy to install as open source web applications like Drupal, Joomla or Oscommerce, but with a little bit of server setting knowledge, and root access to your server, you should have few problems. Installation on a shared server could be a different story. Here are some posts describing some problems, solutions, and addtional info:
Adding Google Analytics to an ELGG Installation
ELGG Social Networking on Shared Hosting
I have a demo up here at http://www.fanclubhelper.com. Feel free to join, set up a community, customize your homepage, and generally test it out.