At least two open source social networking platforms based on Ruby on Rails are out there (if you know of others, please post a comment).

Insoshi is by Michael Hartl, author of RailsSpace: Building a Social Networking Website with Ruby on Rails. LovdbyLess is by less everything, and has a great story behind it:

Along with having our own products, we also develop websites for other companies. Last year we had more requests for a “social network that also does _________” than for anything else. Most of these projects were crippled with a budget so small that they would never be able to afford a well programmed site that meets their goals. So we said we should just build an open source social network platform and give it away.

Insoshi Features

• Activity feeds
• Profiles with photos and comment walls
• Connections/friending
• Discussion forums
• Blogs with comments
• Messaging, including read/replied/trashed
• Search for profiles, forums, and messages
• Admin panel with site preferences

LovdbyLess Features

• Follow a user, mutual following is friending.
• User-to-User Messaging
• Profile Comments
• User Blogs with Comments
• Photo Gallery with Captions
• Site Search for Friends
• Profile Bio and Information
• User Dashboard (Recent Activity of Friends)
• Emailed Activity
• Flickr Integration
• YouTube Integration