If you have a bunch of friends on it - it's actually pretty cool... easy to keep in touch with people - especially since people write about what's going on in their lives (and I mean on a level more than "omg this tv show is so boringggg")
It's also useful for following musicians and getting news info... and contests and stuff.
It's whatever you make of it, but it certainly isn't useless (or pointless) if you are following the right accounts and engaging in it.
Twitter is simpler than facebook. Just brief comments and replies from what I can tell. Signed up to follow my favorite bands. As Hayley said, it would be useful mostly if you have a lot of friends who use it. I prefer facebook because that's where my peeps are and I like the format.
I love it just because it keeps me up to date on things going on with my favorite bands, authors, etc, as well as things going on around town or at some rock clubs or something. A lot of funny and interesting stuff is posted also if you follow the right people. All in all: it's great entertainment.