34 percent of Twitter users have never even made one Tweet.
A few of the other stats:
-74 percent of Twitter users have under 10 followers.
-60 percent follow less than 10 people
-34 percent of users have more followers than people they are following
-Growth jumped 21 percent in April 2009, when Ashton Kutcher was on the verge of hitting 1 million followers.
Which group Tweets the most? Let's check the chart:

Those with around 700-1000 followers seem to Tweet the most, with celebrities and those with millions of followers not Tweeting nearly as much.