One that relates to your interests. I'm on a site called Punkymoms, that's nice, young, different mothers like me, and there's one I used to visit called poetry connection back when I wasn't too lazy to write poetry. Facebook's fun for hooking up with childhood friends, and myspace...I'm totally over it. I was on one called faceparty for a while 'till it occurred to me one day it was basically a dating site, so I left it. Stumble Upon, while I use it every day, has a whole social-networking aspect I haven't even gotten into at all. Same with Yahoo. I have a 360 blog, but it's mostly just for showing photos to my sister.
So, yeah, it depends on you. My computer-nerd brother is a member of all kinds of computer-savvy groups. I even joined a group for fans of my favourite author! The possibilities are endless.