So I've been sucked into Orkut, just like the rest of the geek world, and so far I'm impressed.
It's quite a bit faster than Friendster, and it seems to have a better interface. It'll be interesting to see if it holds up under increasing load.
I must admit that part of it's charm is the massively geek-heavy population. I keep running across people I know from one open source project or another, which is cool.
As for the whole "what constitutes a friend" question, I'm going with the someone you would buy a beer for if they were in town definition, which seems as good as any other I've heard.