As far as I'm concerned, the only useful service Facebook provides is event scheduling. I don't have a Twitter account. Most of the "needs" they fulfill are either entirely imaginary, or aren't fulfilled in any sensible way. (I.e. the whole notion of "staying in touch" with people via Facebook is a sham. It's more of a simulation of staying in touch.)

If you use popularity as the main measure of usefulness then yellow press is useful too.