Who Is Using Twitter, How Often, and Why?

Eight percent of U.S. Internet users are on Twitter.
More than 20 million U.S. adults access Twitter at least once a month and more than 170 million tweets are sent every day.
Younger users (ages 18-29) are more likely to use Twitter than older users. The service is also used more heavily by those with a degree or at least some college education (9 percent) than those who only have a high school diploma (5 percent).

http://www.theatlantic.com/technology/archive/2011/07/infographic-who-is-using-twitter-how-often-and-why/241407/