From a Harvard Business School research study:

(1)  The median number of tweets over the course of a lifetime is…. ready for this?… one.  This translates to over half of Twitter users tweeting less than once every 74 days.  Yup.  In fact, 25% of all Twitter uses don't tweet at all (they just follow).

(2)  90% of all tweets come from only the top 10% most prolific users.  Whcih means 90% of all Twitter users account for only 10% of all tweets.

(3)  Men comprise 45% of all Twitter users; women are 55%.  But both men and women tweet the same amount (no significant difference).

(4)  Men and women roughly follow the same number of twitterers; but men get followed more often than women.  In fact, men have 15% more followers then women, even though there are less men on Twitter (see #3).

It should be noted that the data for the study came from May 2009.  There's been a Twitter explosion since then.  I'm not sure how much the underlying numbers have changd — I suspect that #1 and #2 will be different now.