During Thursday’s quarterly earnings call, Facebook’s CEO Mark Zuckerberg said people on Facebook in the U.S. “spend around 40 minutes each day using our service.” That’s about 280 minutes a week. Of those 40 minutes a day, 20% of the time is spent accessing the site via mobile devices, he added.

The Centers for Disease Control and Prevention (CDC) recommends Americans spend a minimum of 21 minutes each day doing physical exercise, such as moderate-intensity aerobic activity, or 147 minutes a week. In reality, very few actually do: Only one in five adults meets overall physical activity guidelines, or 20% of the population. That doesn’t include muscle-strengthening activity, including push-ups and sit-ups.

While the CDC hasn’t explicitly studied Facebook’s effects on physical activity, a spokeswoman said that the government organization recommends limiting screen time, especially for children. Too much time on a computer may cut into precious time spent outside getting the daily recommended dose of exercise, the CDC warns.

Zuckerberg’s take on Americans getting more screen time, however, is much different. He said on the call that people in the U.S. spend nine hours with some sort of digital media, including television, phones and computers, during the course of a day. This, he said, equates to a “big opportunity” for Facebook to expand past that 40-minute mark and to “improve the way people can share.”

And while those extra minutes may get Facebook more cash, they could mean Americans spend even less time exercising. Not good news for a country already struggling with obesity.