A lot of people do keep in touch. I love birthdays/major occasions in our group of friends because people will always invite their other mates. As we're about to graduate, it's got to the stage where we've met some of them enough times that they're considered our friends too and not just ''X's mate from college''.
We're lucky that a lot of people haven't moved too far away from where they lived before, so it's easy for them to invite others to stay. Obviously that'll be different if you're from say, Plymouth and going to university in Newcastle. I don't think it's any coincidence that the 2 people in our group of friends who haven't brought any of their other mates down are the ones who live the furthest away.
It's the same with anything really, finishing uni, leaving your job/sports team etc. You'll keep in touch with a couple of people you've built a close relationship with, and then the others who were more mutual friends/friends via circumstances you won't keep in touch with but as soon as you see each other again, it'll be like nothing has changed.