Because it's an extremely good, extremely efficient communication tool for keeping in touch with people you know. Much better than everything else there is so far.
I don't have a facebook profile and I keep in touch with people with the exciting new technologies of email and telephone. Amazing what actually talking to people does.
I also use the exciting technologies of email and telephone, and talk to people. On top of that, I use Facebook to keep in touch with the remaining 90% of people I have weaker ties to, but once every half a year or sth. want to do some stuff together.
One big value of Facebook is that it's a push system, not pull (and we know that push > pull). I can stay current with the life of hundreds of people I know without having to think much about it or check it manually. It scales better than e-mail/phone and allows different new ways to interact with people. And there's no reason not to use e-mail and phone along with using Facebook.
As someone with the same email address for fifteen years, the same phone number for almost ten, and who deleted his facebook account five years ago, I can now say that facebook does it better.
I made a new account this month and reconnected with people I haven't been in touch with for years. Not everyone is going to set up a mailing list you can subscribe to.
