A certificate-based trust metric has already been implemented, in the form of Advogato. You need to be vouched for by existing members, and you gain ranks as more people certify you. Bad nodes can be separated and isolated, protecting the entire network. Orkut may offer some new features, but so far I haven't heard of anything groundbreaking in it's design.

Secondly, I don't see the point of all these Friendster-like websites. I manage my friends quite well without my relationships being formalized via the Internet. Seeing how I'm connected to other people doesn't interest me one bit. Maybe I'm weird, but it just seems ... unnecessary.