Awesome article on finding old lovers on facebook.(TIME.com) They are calling it "retrosexual".

excerpt:
Most retrosexual experiences seem to spring from an intense, almost uncontrollable mixture of nostalgia and interest. "You get a thrill out of finding an old girlfriend just to see if she still likes you," says W. Keith Campbell, a University of Georgia psychology professor and co-author of The Narcissism Epidemic. "You're curious to see what she looks like, and it's easy to fantasize about alternative courses your life might have taken." It's the same feeling that compels people to attend high school reunions.

Full Article

I have to admit this happened to me, sort of. A person I grew up with found me on fb. We never dated but had a long, (sometimes flirtatious) friendship and even made out a few times as teenagers. We had seen each other since all of that but had not hung out in maybe 7 years.
He came to my birthday party a few weeks later and we started hanging out all the time. Our intense nostalgia for each other and ease with which we picked up our old friendship culminated in a slightly drunken hook-up. I will spare you the details, but we are no longer friends. For me it was a lesson that some things are better left undone.
Curiosity killed the cat people!