This has been one of the things that has really upset me too over the years. I'm in my early 20s and am a really independent guy, but still I really miss my old friends. We used to hang out all the time in highschool, talk daily. They are moving away, or becoming more distant, or hooking up with gf's/bf's and spending less time with old friends.
It seems to happen. The good news is that there are always new friends to be made. More good news, if you put in an effort to meet up with your old friends (go visit them, invite them over, etc) you'll probably find they are really receptive to you. It reminds you that you are still friends. I recently met up with some of my oldest friends, from like 15 years ago, and it was a really fun time.
As for friends that seem to become more distant, put in an effort to keep in touch with them but if they are withdrawing from you then I know it's sad and I have no idea what you can do about that.
I wish people all could put in more of an effort to keep their long term friendships. Social psychologists like Zimbardo observe that the overwhelming factor in depression, loneliness and loss of direction in Americans' lives is losing touch with friends and close contacts. Whenever you can, try to keep the lines open with people you think are friends. Call them up, talk. It really can't hurt, and it will probably make everyone feel better.