The most active 10% twitter users post 90% of all tweets

"Specifically, the top 10% of prolific Twitter users accounted for over 90% of tweets. On a typical online social network, the top 10% of users account for 30% of all production. To put Twitter in perspective, consider an unlikely analogue - Wikipedia. There, the top 15% of the most prolific editors account for 90% of Wikipedia's edits ⁱⁱ. In other words, the pattern of contributions on Twitter is more concentrated among the few top users than is the case on Wikipedia, even though Wikipedia is clearly not a communications tool. This implies that Twitter's resembles more of a one-way, one-to-many publishing service more than a two-way, peer-to-peer communication network."
Source: Harvard Business Review blog 1st June 2009