Facebook is a place where you go to chill and waste time, connect with old friends, check pics of old crushes, catch up on gossip, play games, or share links.
The last thing you'd do there would be to search for another student, it's not built for that.
StackOverflow is basically a high signal-to-noise ratio content farm. It sells the illusion of fame in exchange for generating content. There is no meaningful social aspect to it. I don't see any signs of it evolving over time, as it seems to exist for a very specific purpose.
A chat system, a 'twitter' like system, a limited blog with comments system, photo sharing with comments, and the familiar followers/contacts that many site have these days.
While amazingly successful, I'd hesitate to call Facebook 'a technology'. They haven't pioneered much except for jamming all that stuff together for a very large audience..
Alternatively: Facebook is a collection of wide-ranging social technologies combined in a tasteful, approachable manner, which has captured the interest of a wider range of the general public than anything before it, by a wide margin.