(Reuters) - The average time spent accessing Facebook via smartphone in the United States was 441 minutes in March, compared with 391 minutes via computer, according to comScore, underscoring the increasingly high-profile role of mobile in social networking.

comScore's new Mobile Metrix 2.0 report showed U.S. smartphone users spent 441 minutes per month, or 7 hours and 21 minutes, on Facebook in March. That compares with 391 minutes, or 6 hours and 31 minutes, for people who tapped into Facebook via a computer.

In filing documents for its initial public offering, Facebook highlighted the importance of mobile while noting it does not generate meaningful revenue from mobile users.

"If users increasingly access mobile products as a substitute for access through personal computers and if we are unable to successfully implement monetization strategies for our mobile users," the company writes in its filing documents, "our financial performance and ability to grow revenue would be negatively affected."

Beefing up its mobile strategy was part of the reason Facebook in April agreed to spend $1 billion to buy Instagram, a photo-sharing mobile app, analysts say.

Historically, Facebook hasn't shown ads to mobile users, although in March 2012 it started including "sponsored stories" in users' mobile new feeds.

March marks the first month comScore measured mobile usage -- for Apple's iOS, Google's Android and RIM's Blackberry -- on sites like Facebook.

Facebook commands the lion's share of smartphone users' time, comScore data showed. The next-most popular services were check-in services Foursquare, with 146 minutes; microblogging service Twitter, with 114 minutes; and blogging-service Tumblr, with 68 minutes.

Facebook likely racked up more minutes because people like to stay on to craft updates, read friends' updates, and respond, said Michael Pachter of Wedbush Securities. By contrast, "it doesn't take that long to tweet," he said.

comScore previously reported that Facebook computer users spent 422.8 minutes on Facebook in December.

But it says the March figure of 391 minutes doesn't represent a drop because it is now calculating the data differently after learning it had been double-counting users in some limited instances due to certain sites alerting servers twice when users got on the sites and used Facebook plug-ins. It has now fixed the glitch, a spokesman said.

The March numbers represent a rise of six minutes over February, when computer users spent 385 minutes on Facebook, comScore said.

Facebook's rich valuation -- approaching $100 billion at the high end -- is in part based on high levels of user engagement. Facebook cautioned in its filing documents that as growth in its numbers of users slows, its "business performance will become increasingly dependent on (its) ability to increase levels of user engagement in current and new markets."

comScore said Facebook has about 158.9 million unique U.S. visitors who access the site on computers and 78 million who access it via mobile phones, although there is overlap between the two groups.

Facebook declined to comment on the study.