I have just discovered Twitter and must confess it is kind of sticky.
I especially enjoy the ability to post through via mobile phone. Unlike other IM, your message || status || tweet remains there when your computer is off. Very quickly, you'll send you invitations to your friends, and search Twitter for friends or people you know. The interface is really simple. All messages (or tweets?) are SMS size length, meaning they are limited to 160 characters. So be inventive.
Once you've added friends, their tweets show up on your home page, a bit like this: (Flash 9 required, sorry. Found it on the Twitter Badge page)
Flag your favourite tweets as favourites, send a direct message to one of your friends (both parties must be friends), receive notifications on your IM client or mobile phone.