It seems like a week doesn’t go by without another second screen study — here’s one from last week — but Nielsen’s reports are watched very carefully within the industry. Nielsen’s Cross-Platform Report (.PDF) is out, and it provides even more solid evidence of the growing proximity and overlap between devices and televisions.

“Close to 40 percent of Americans now use their tablets or smartphones while watching TV at least once a day, and twice as many (85%) do it at least once a month,” the report concludes, adding that the behavior is prevalent across age groups. “These devices are omnipresent and not just among the young.”

While email remains the most popular multitasking activity, 44% of 18-24 year olds and close to 50% of 25-34 year olds are visiting social networking sites on their smartphones during both commercials and programs while watching TV. With tablets, 36% of people 35-54 and 44% of people 55-64 dive deeper into the TV program they are currently watching, the report says. Here’s the frequency breakdown by demographic:

The study also found a small decrease in the total time spent watching live TV, with DVR playback gaining slightly for the second quarter of this year.