The Twitter analytics team is sharing some of their usage data and its a beautiful example of exponential growth.

Folks were tweeting 5,000 times a day in 2007. By 2008, that number was 300,000, and by 2009 it had grown to 2.5 million per day. [ ... ] Today, we are seeing 50 million tweets per day—that’s an average of 600 tweets per second.

Tweets per day is just one number, but a pretty interesting one to look at.

Twitter Blog: Measuring Tweets.