My reader has been active. He has sent any number of interesting Twitter analytics pieces to illuminate the distribution curve question. This one is of particular interest. http://www.techcrunch.com/2009/10/05/twitter-data-analysis-an-investors-perspective/
Among the more revealing stats is that only 1 in 4 registered users tweets in any given month and that 75% of users have tweeted fewer than 1o times and that substantially less than 5% of users have more than 100 users.