An eMarketer study has found that U.S. adults spend an average of 21 minutes on Facebook per day.

That certainly seems modest compared to, say, the five hours of television Americans watch per day, or the fact that it only accounts for 2.8% of the 12 hours and 28 minutes that U.S. adults will spend engaging with the media at large every day in 2014.

emarketer chart This chart shows the Facebook habits among US adults as a whole. Photo: eMarketer

But the data is the average of the entire U.S. adult population, 47.2 percent of which will log in to Facebook less than once a month -- or not at all. The data show that 52.8 percent of U.S. adults, or 129.5 million people, log in at least once a month with varying degrees of engagement. 

Data from eMarketer shows the latter group will spend an average of 39 minutes on the site per day, which represents 38.1 percent of their total social networking time (see chart below).

emarketer chart These are the Facebook habits among US adults who use Facebook more regularly. Photo: eMarketer

In addition, eMarketer found that Facebook’s business occupies an “unusual position in the digital environment,” largely due to users' ability to tune out or otherwise skip ads on other social media and entertainment channels, including Pandora and YouTube. From eMarketer:

"In 2014, US adults will spend 7.1% of their daily time listening to Pandora, according to our figures. That’s not a misprint: For the average US adult, daily time spent with Pandora exceeds daily time spent on Facebook. However, advertisers will only allocate 1.4% of their digital ad dollars to Pandora, a fraction of what they devote to Facebook."

The research firm said marketers favor Facebook for a number of reasons.

"Several factors contribute to Facebook’s unusual position in the digital environment. For example, Pandora is often on in the background, and users can tune out the ads or simply not hear them. Similarly, while digital video is an engrossing activity, viewers can easily ignore or skip ads, and targeting the right viewers is still challenging for marketers in the nascent medium."

In other words, you may be spending too much time on Facebook.