I started on Twitter ( http://twitter.com/drewb ) last month but it didn't work on my phone. Twitter works by you texting your brief comments and life updates to the system which then publishes it to your online account, to your online social network, and to their mobiles if they want. When it's not working, or when you've not managed to add anyone to your network, or if you're not on it, you kind of wonder what the point of it is. But when you get it working, you're hooked.
Bobbie Johnson first got me interested a few months back. Then Scoble did it, then Sam Sethi when he left TechCrunch, Mike Butcher and more, so I tried again. Turns out the reason I couldn't get it to work is that my mobile phone isn't compatible with the Twitter system. Rotten huh. So I've installed an IM client on to my blackberry. Now it's working super. Only problem is, I've got IM on my mobile - how dangerous is that going to be!!!! Not only am I 24-7, but all my buddy list (that's work- and buddies-buddies btw) can see when I'm online too :(