Numbers for Facebook's mobile service have already proved that it's popular, but new reports from comScore show that people are actually spending more time on mobile devices than on the desktop site, at least in the US. According to the Mobile Metrix 2.0 report released yesterday, people who use the mobile app or site spent an average of 441 minutes (about 7 hours and 20 minutes) using it in the month of March. By contrast, US users who accessed it from a computer spent 391 minutes (about 6 hours and 30 minutes) on Facebook. Both of these numbers are growing, comScore says, although a change in the way the company calculates usage means that the number for computers is shown as lower than it was in December.

Although Facebook enjoys a far higher average mobile use time than any other social network — the closest runner-up is Foursquare, with 146 minutes — it's expressed concern about monetizing the service. As Reuters notes, Facebook said in its IPO filing that "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" it could negatively affect revenue. Facebook recently started adding sponsored stories to the mobile news feed, but users are still shown far fewer ads than on the desktop site. App integration, on the other hand, has proven to be extremely successful. Despite these numbers, however, many more people are still accessing the site online. ComScore says that about 158.9 million people in the US access the site via traditional computer, compared to 78 million mobile users.