However, the free microblog site Twitter is worth monitoring. Microblogs, the buzzword for short messages that basically amount to a soundbite about your current activity, are something that has been around for a while. Many sites, including NewsBusters, use it as a means of putting out information that isn't worth it's own blog post. Facebook, though, probably brought microblogging mainstream.
In any event, if you're interested in social media, you should read this interview that Mark Glaser conducted with the founders of Twitter, a site devoted exclusively to microblogging. He explores many different topics including the company's different management style, whether they even have a business model, and text messaging in the U.S. Must read.