According to the research, conducted on a random sample of about 300,000 Twitter users in May 2009, 25% of Twitter users don't tweet at all, while 50% of users tweet less than once every 74 hours. Active users, on the other hand, tweet a lot, which makes Twitter a lot more like Wikipedia than an average social network (see graph below, courtesy of HBR).
Although this may sound strange at first, Twitter really is more like Wikipedia than, say, Facebook. Twitter is not so much about connecting with your friends, it's about broadcasting information. Although it doesn't necessarily take much creativity to create a tweet, only the most creative users actually persist in tweeting every day over a longer time period.
However, Twitter is also similar to a instant messaging tool, which should have a very different curve, with a larger proportion of users contributing to the number of overall tweets. It seems that Twitter's micropublishing component is winning over its chatting component.