What's the best social networking platform to build a site on?

I'm sure this is one of those headaches that site owners are thinking about when taking on web 2.0 . Do they use Myspace, Facebook, Linkedin, Bebo, YouTube, Ning or stand alones. I was surprised only to find one company offering social networking build sites at Adtech in London last year, Perhaps there will be some more at Internet World 2008 this year. What would anyone else use?

Found this US one I'd never heard of before Pringo - I'll take a look at it and report back.