From the outside, Twitter looks really ridiculous and pointless. It wasn't until I signed up and started following people that I really got it.
It's not just a social networking site the way Facebook is. You can barely customise your profile, and your updates will quickly move down the page as everyone else's come in, so there's no room for ego. It's not really about you or about any one person; it's about what a whole bunch of people are doing and thinking right this minute.
It's like plugging into a kind of goodnatured chattering hivemind. If you only go onto it to post your own updates, and only follow people you know, you won't get a whole lot out of it. Sign up, follow Stephen Fry, Graham Linehan and Rainn Wilson, then look for your friends. Then follow your friends' friends, and whoever they're following, until you're following about thirty people at least. Then watch them go about their day. It's kind of hearteningly hypnotic to watch thirty people's minds tick over. Some of them think about the same stuff, some of them think on completely opposite ends of the spectrum, but it's a fascinating snapshot of what's going through everybody's head just now.
The celebrity part of it is, I have to admit, a big part of the attraction. Christopher Walken's Twitter, although clearly fake, is freaking genius. Jimmy Carr's really is Jimmy Carr, and is a genuinely brilliant look into what it's like to be Jimmy Carr. Even if you hate his comedy, I can't recommend his Twitter enough, it's fantastic. And I can't help it, when somebody I idolise responds to me, I do grin like a giddy fool.