So who is using Twitter?

Twitter Traffic Explosion: Who’s behind it all?: "The gender and age breakdowns indicate that users skew young and male. These two charts are indexed to the U.S internet average, where “100” is the average.
Twitter users are 10% more likely to be male than the average internet user. This skew is nearly identical across all three Usage groups.
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.
Twitter may be growing like crazy but it’s yet to go mainstream. Interestingly, Twitter’s strategy of letting developers re-purpose its data may help the service reach the tipping point by making Twitter’s value more accessible. Check back next week to see how these twitter based sites (like twitterlocal and summize) are building a loyal base."