People I think about a lot who have gone missing-in-action from my life (mostly due to my being an idiot and not keeping up on correspondence):

Kim — She's off teaching at a University on the prairies these days
Alexei — He's off in China.
Diana — She's married, in Ottawa, with 4 children (3 being step-children). Still changing the world.
Eric — He moved to California after I left Pittsburgh. His email address stopped responding a year later and I don't know where he is any more.
Dan — Married a very strong-willed woman. Disappeared to South Carolina a day later. No one seems to know what became of him.
Nadia (1) — Lost her sight. Moved back to Montreal to live with her family.
Denise and Peter Moberly, Mohammed and Paula — I left Waterloo and immediately lost contact with them.
Nadia (2) — Decided she didn't want me to see her dealing with various medical problems, so broke off contact.
Bevin — Went to the other high-school. Only saw him once after that, years later. Didn't really have anything to say to one another by then, he had new friends, and I was well into my outcast and unhappy phase.
Toria, Eva — Disappeared at the end of high-school. I was an idiot and never bothered to keep in touch.
Visser twins — They're both married, with children IIRC.
Tin, Mike — Lost at the end of high-school, like everyone else at school.

I really have to start working on maintaining relationships.