In case you missed Harvard Business Publishing’s survey of Twitter users:

Here is what they found: (Of their sample of 300,542 users collected in May 2009)
–The top 10% of prolific Twitter users accounted for over 90% of tweets
–Among Twitter users, the median number of lifetime tweets per user is one
–80% are followed by or follow at least one user
–Men have 15% more followers than women
–An average man is almost twice more likely to follow another man than a woman
–An average woman is 25% more likely to follow a man than a woman
–Men represent 45% of Twitter users
–Women represent 55% of Twitter users