I like Twitter. The idea behind it is you post a short message about what you are up to and it then appears to anyone who has subscribed to your updates. It takes my favourite feature of Facebook, status updates, and then removes all the other crud.
The nice thing about updates (or "tweets" if you want to look like douche) is that they are passive. They don't fill up an inbox like email so you can let others know about something you are up to that is interesting, but is perhaps not worth spamming your entire address book with, e.g. "Working on my first MVC application. Looking at getting started guides." It is a great way to start conversations that never would have happened otherwise.
Twitter is a nifty service but it ia not perfect. The Twitter website is flaky (perhaps not the best showcase for Ruby on Rails) and the web UI doesn't handle subscribing to a large number of people particularly well. I think at the moment the place where Twitter shines is with small communities.
p.s. I have added my Twitter status to the blog (look to the right). A great example of a useful JSON API [:)]