As much as I really dislike FaceBook, I must admit that I have been able to use it to improve my photography business. It has uses. That said, it absolutely amazes me that anyone, repeat anyone, gives a rats turd about the day to day minutae of other peoples lives. I'm really not a people person though.
My wife spends a fair amount of time on FaceBook, and now Pintrist... She enjoys it in her spare time, who am I to say that it is bad. To each his own, I guess.
Twitter, I find totally useless.