You're going to get a lot of people saying Myspace, Zayne. Even I have started using it a bit more. It's a glitchy site that's not really state-of-the-art... but there are lots of people there, and that's the point of a social networking site: to meet and talk to people.
I guess you could count Gaia Online as a social networking site, even if not in a traditional sense. I go there very often. I put a link back to here in my profile too, so hopefully some people will click it and sign up here eventually.
Does Pownce count? I'm trying to figure it out. I might start using it more now. Jaiku is awesome too. But I just don't know anyone at either place yet, so I haven't been to them often.
I'd say Digg, but that isn't really social networking. It's more of a tech-focused news site.