Sell on facebook and other places. They handle everything for 7% plus 3% + 30c for card processing, but they'll take cards for you, no merchant account required, through elavon.
Community Server is a white label social networking platform. It runs on Microsoft .NET / IIS technology. It powers the MySpace Forums and Microsoft's adCenter. Starts from $3k per year (small site), $50 per seat (min 200 seats) or $20k per server.
BuddyPress is a series of plugins which transform a vanilla WordPress MU installation into a social network. It's due for final release around Dec 2008.
Social Engine is a white label PHP based social networking platform. You can implement your own plugins around their architecture. Basic costs $300 plus $290 for all the plugins ($30 or $40 each) as at June 2008.
Ping.fm allows you to update a bunch of social networks in one go. Post to ping.fm and have it passed on to Twitter, Facebook, plurk, myspace, linkedin, plaxo, bebo, etc, etc.
An article outlining social networks that target the rich. aSmallWorld is apparently the most popular, where only certain members can invite new members.
KickApps is a white label social networking / video hosting platform. Free to sign up, ad supported, or you can buy the ads out, starting at $100/mth for 5'000 page views.
Ning products including removing their ads, their links, your own domain, and extra bandwidth. The full suite costs $33/month plus $10/month per 10Gb disk / 100Gb bandwidth.
BrightKite is location based social networking. It will post your current location to twitter, and other cool stuff. Currently invitation only (unlimited invitations).
Moggl is a social networking site. I met a developer who works for moggl at a Cape Town GeekDinner.