Yea it happens probably more and more as you get older. It could be because you find people that you are more comfortable with or just busy schedules (work or different classes).

I agree that rather than having a friendship crushed by a fight, a lie, jealousy etc drifting away is better...
But yes, its happened once or twice, but that doesn't mean that you were never friends, or you never meant anything to eachother...
When I switched schools, I lost all but one of my friends, and the only reason I was able to keep my relationship with her was because we live close by to one another.

I had a friend that I've known since pre-school. We played basketball, softball, danced hula, and went to school together for nearly 12 years. She got more into sports and I went into orchestra and band. Our colleges are near each other and we're trying to meet up again, maybe even start dancing again :)

Yes, but it wasn’t a particularly close friendship. We were in English class, and we became cool. We got along fine for the semester, but the next year I saw him, he ignored me. He was a really popular kid, which was odd because I wasn’t really unpopular myself. I got along with lots of people, many with status. Oh, well. The next time I saw him, he was going to work at a fast food place, and I’m heading off to college!

...............

I also think is common nowadays, the reason of why your are with someone is because you have common topics to talk about or have mutual experiences to remember. But as you get separated from your school friends (going to different classes or something) then you start to have a hole of knowledge from your friend, they have different homeworks/exams/asignatures and its more difficult to go and talk to a person who has less common things with you. As they are with other people, they will tend to have different thoughts and the hole will get bigger.
I don't know anything about any of my school friends..

Yes, that has definitely happened to me.
In grade 5, I was best friends with this girl; we were inseparable!
However, she made another friend, and I didn't approve of the friend, because I knew her. So I unfortunately stopped hanging out with my best friend, and we didn't try to make it up. (I think this is what happened anyways, it was a long time ago, haha). It's sad and it makes me feel stupid when I think back to the incident, but the experience surely taught me that friendships should be clung onto if they are good friendships. However, life will always be unpredictable, and sometimes friendships are just lost, and nothing can be done. If a broken friendship can be healed, then by all means try to make it up with the other person; if not, then just move on with your life!