From the NYTimes…

In the fourth quarter of 2007, American cellphone subscribers for the first time sent text messages more than they phoned, according to Nielsen Mobile. Since then, the average subscriber’s volume of text messages has shot upward by 64 percent, while the average number of calls has dropped slightly.
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Nicholas Covey, director of insights for Nielsen Mobile, attributed the spike in messaging to the spread of QWERTY-style keypads, whose users send 54 percent more text messages than those with ordinary keypads. He also said that phone companies had encouraged users to text by offering large or unlimited text-messaging bundles.

Teenagers ages 13 to 17 are by far the most prolific texters, sending or receiving 1,742 messages a month, according to Nielsen Mobile. By contrast, 18-to-24-year-olds average 790 messages. A separate study of teenagers with cellphones by Harris Interactive found that 42 percent of them claim that they can write text messages while blindfolded.