Why? It's one the best ways of keeping in touch with people. MSN Messenger is good, this is a as good, if not better. The beauty of it is its user-friendliness and it's really easy build a network of randomly connected friends. Picture-uploading is awesome, and shite easy and wall-posting is brilliant. It's easy, not complicated, and very, very addictive. It keeps you updated on everyone you want to be updated on even though you are too scared to admit so. :P
Not convincing enough? Ask anyone who has a Facebook! In short, get one, it's worth it! And add me. :P
P.S. Merry Merry Christmas to everyone, especially to Jesus' followers.