Facebook is the bastard son of myspace and friendsreunited. Handy for looking up ex girlfriends and for reminding yourself why you stopped talking to the people you went to school with in the first place.
Many of the apps are pretty crap, little more than web 2.0 "tag you're it" style nonsense involving the usual cultural niches (Pirates, jedis, vampires, zombies), but there are exceptions (Scrabulous for example)
Also Conspirisist theories point to the fact that somewhere down the line its owned by the CIA and is probably used to profile everyone on the planet, I mean why bother with the hassle of actually sending agents out and collecting this info if you have a website that will do the legwork for you?
That said there is something rather good about just seeing what people you know are up to, the news feed is cool, but the novelty value of the site is beginning to wane.