This is usually done with Tweets containing links, so naturally it provides a good, viral means of link exposure.
Tweetmeme has been around for a while, offering a service to content providers, where they can add a button onto an article page that lets a reader easily tweet a link to that article on Twitter. It then counts these tweets, which become retweets, just like similar buttons youve probably seen for Digg. The more retweets that are registered on that button, the more interesting the content looks at first glance. The reason for this is that theoretically, if a user sees the article has 2,000 tweets, as opposed to 2, they can assume that a lot of people found the article interesting or informative, and will be more likely to continue reading.