A while back I wrote a post about Twitter and how I didn't really like it. This was mainly from a personal perspective, where it was just me and my friends on Twitter and people would just tweet about random stuff.
These days however I've been using Twitter as an advertising tool for the Pirate Party of Canada and it is really working well. I tweet about a meeting or an announcement, and people retweet it, and the news gets spread really quickly. Compare this to Facebook (well Facebook now has a more Twitter-esque model on the front-page but we don't really have a Pirate Party page on the site other than the group) where you can't really broadcast a message easily to lot's of followers. Well, you can send a message to all the people in a group, but there are a lot of people that the message doesn't apply to and it's probably annoying for some people in say, Vancouver to always be receiving messages like "there's a meetup in Saint John, NB". Whereas on Twitter you can just ignore the tweet.
So in summary, Twitter is a pretty good communication tool for a one-to-many broadcast. In case you hadn't figured that one out already :)