You know how Facebook has a little clause that says people can't have multiple accounts? I of course expect them to shut the accounts down, when they find out... but how do they find out?

Is it through the IP address? That say, 5 people all have the same IP? But what if that's a public computer and people just share it?

I'm just curious, is all. Or do they really not know? Or maybe wait for other people to tip them off?