Isn't it funny how you spend four or six or even ten years of your life with the same people, seeing them five days a week for nine months out of the year--and then, when you move away, you rarely talk to any of them, ever again?
Friendship is more situational than we'd like to admit. Now and then we meet a friend who will be with us for life, but for the most part, out of sight is out of mind. The Internet has certainly made it easier to keep in touch with distant friends, and this is probably a good thing. But it could be that the reason long-distance friendships tend to fade is that your old friends originally made friends with a different person. Separated geographically or ideologically, friends don't get a chance to create new bonds and share experiences. Friends who do not grow together, grow apart.