I remember being at school and being the social outcast because I spent my evenings on the internet. Now I'm the social outcast because I'm not on the right part of the internet. People I used to hang out with stopped even sending texts of "Coming down to the pub?" or even just stopping by my house on their way past, saw them there one day by chance and they asked where I'd been and why I never came out any more, "we always post on facebook before coming out!". Once they adopted it they just assumed all others had.
Personally, I dislike it as an incredibly passive way to keep up with your friends. Log on, read a list of what usually consists of status texts, update your own, log out. Gone are the days of meeting someone and asking what they've been up to, you already know due to 5 word tags. I really dislike the fact that I can be tagged in pictures that other people have uploaded. I could see it have a use for reconnecting with people you've lost touch with (particularly if it was on bad terms, as the "someone you know" alert can help you determine if they are receptive without actually messaging them) but that's probably it.
It's an incredibly intrusive website, for a long time it was impossible to DELETE an account only disable, iirc they were legally forced to change it. Any website with a facebook 'like' button on it will access cookies, and track where you have been, feeding the information back to facebook, which can then be combined to other websites that have tie-in's, facebook from phones can use this to actually map your whereabouts in the real-world and feed it to tracking. There was something a few years back if I recall that they were scanning every private message for a mention of failbook and blocking the entire message from being received which got them quite a bit of flak. I think the worst I've seen reported was that if someone tried to friend you, they changed the options to 'Yes' and 'Not Now', there was no decline option, and the 'Not Now' still allowed viewing access to the majority of your profile. Blergh, it provides a lot of entertainment to a lot of people so I cannot condemn it, but I certainly won't be using it any time soon (which will probably please employers).