Compete did an analysis of Twitter users in April. They found users tend to be Young, Male, and Twitter Addicted.

Two fascinating stats:

• Twitter skews heavily towards the college/twenty-something crowd. Twitter attracts 18-24 year-olds at nearly twice the rate of an average U.S website.
• Splitting age demographics based on usage intensity shows that heavy users tend to skew older than visitors who only hit the site once a month. This could indicate that while the younger segments are more exploratory, the 25-44 year old segments have found more value in Twitter and started to ramp up usage.

(via Beth Kanter via Leslie Bowden Twitter)