So is life. The longer you live it, the more people you'll know and the more people you'll have passed on by...
It's very true. I have had tons of friends move away, but the truth is, some of this stuff is for the best. New people will arrive, and they can turn out to be neat people. I'm not saying replace your friend with the first stranger that happens to move over to your area, but keep yourself open to options.
It's ok, keep in touch using the phone or internet if you can
People move away all the time nowadays unfortunately. Just keep your head and keep trucking, new friends are a dime a dozen, even the best ones.
It is sad but some friends can always keep in touch. My friends from school- Well I moved country for University and mostly lost touch but I still see the closest when I can. After Uni we all spread out though the country so its hours driving between us (this dosn't seems like much but with full time jobs its very annoying.) However now our group of friends from Uni sees each other loads and chats online- we all get on and play halo together a couple of times a week to stay in touch and all meet up in person when we can. Soon I'm going to move to Canada though so that will complicate things further... but you can always keep in touch with the ones you need to, and others will come and go throughout your life, you will just need to learn to accept that some friends will be in your life only for a short time before your paths diverge.