I didn't have many close friends, who I had a lot in common with, at school, so it might be different to what you're asking, but I haven't really kept in contact with many people from school. The girl who was probably my closest female friend has gone to Nottingham and I speak to her on msn and facebook from time to time and see her when I go out, but we're in different friendship groups really. Which is fine, and I see her and my old friends when I go back in the holidays. My best friend in sixth form was probably my boyfriend, and if we speak now it's a bit weird (we split up in January), though he does email me occasionally.
When I'm back at home and people meet up, they're usually going into town at night, so I don't see them that much during the holidays. The friend from home I had the most in common with moved to Devon a few years ago, and I keep in touch with her more than I do the people I knew until sixth form.
I guess what happens is, when you're at uni you don't have to bother making conversation with the friends you only had because they were in your social group anymore. It sounds a bit mean, but uni is like a 'filter' in that sense. At uni you tend to make really good friends with the people you have things in common with and who are more like you, whereas at home, you made friends with those people years ago, and perhaps don't have so much in common anymore.
If you're worried about losing your friends, OP, I think you'll find that when you go back in the holidays everything is pretty much the same as before, except you'll have a whole new, extra group of friends to look forward to seeing when you go back to uni.