Because its dumb. Well, but that's not a good enough reason, heres a couple of 'em.
Stay in touch huh?
Well, if you need to talk to me, there is something now a days called the cellphone. What's that, you do not have x friend's cellphone? There is the e-mail. You want to chat? There is AIM, MSN, ICQ, etc, etc.....
There is no need for Facebook. Want to share pictures and stuff? Flickr is your answer. Want to post that juicy gossip from last party? Nothing like a good blog on Blogger or somewhere else.