What should I write about on Twitter?

I've been using Twitter for a couple of months now, it's a mostly pointless service that I keep going back to. Some days I can think of lots to write but most days I can't think of anything. I think I'm trying too hard to post something useful or interesting rather than just something random.

I'm following a number of people now, some of them post generally interesting stuff thats happening others post crap. 

I'm interested to know what other people use Twitter for and what they get from posting stuff on there? (The same can be said for Facebook status updates).