A separate study, from the Pew Internet & American Life Project, backs those findings. It found that 40 percent of U.S. adults used a mobile device to surf the Web, send e-mail or participate in instant messaging. Those figures from May are up from 32 percent in 2009.

And more people reported taking photos, playing games and listening to music on their mobile devices compared with a year earlier, the Pew survey found.

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USC's telephone survey of 1,981 Americans over 12 was conducted in April 2009 and has a margin of error of plus or minus 2.2 percentage points. Pew's survey of 2,252 U.S. adults, conducted in May, has a margin of error of plus or minus 2.4 percentage points.

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