How does Game Center recommend friends?

So I've had an iPhone for quite a while and never really bothered bothered to open the Game Center app, until a couple of days ago. So I tap on the friends tab and see the recommendations, many of which are people I actually know from college and whatnot. The weird thing is, I don't have these people in my contacts, and I've never once logged into my Facebook or Twitter account on this phone. I know it can't be based on location since some of them live far away from me.

Anybody know what's up? I'm really just curious. My Google-fu has failed me so I figured I'd ask here. Thanks!