It's been entertaining, mostly. It motivated me to go my 20-year high school reunion this Fall :) I've reconnected with some old college buddies. But if FaceBook disappeared tonight, I'd be OK.
I have a friend who joined, went gonzo with it, and the deleted his account a week later. He found too much tension in moderating his comments with his extended family seeing them and having tensions about how to block his employees and coworkers from connecting to him on FB.