BBC story based on Harvard study of 300,542 users in May 2009, which shows that 10% of Twitter users generate more than 90% of the content.  The study also looked at gender differences, noting that men had 15% more followers than women despite there being slightly more females users of Twitter than males.  Additional coverage of the study at MinOnline.  The full report (”Men Follow Men and Nobody Tweets”) on Harvard Business Publishing.