Facebook users spent more than 10.5 billion total minutes per day on the site from personal computers in January, according to an updated S-1 filing with the Securities and Exchange Commission.

With 845 million monthly active users, that’s an average of 12.4 minutes per user per day. The number of minutes per user per day is important to Facebook’s display advertising business. The more time people spend on Facebook, the more likely advertisers are going to be to allocate budget to the platform. According to Facebook, aggregate minutes per day increased 57 percent year over year since January 2011. Since the social network simultaneously expanded its user base, the actual average minutes per user per day increased 14 percent during that period.

The filing does not include information about time spent on Facebook through mobile devices. The company previously noted that had 432 million mobile monthly active users as of Dec. 31, 2011, and that 58 million of these users accessed the social network exclusively through mobile devices. In today’s S-1 amendment, the company said the number of mobile-only users increased relative to the previous year. Facebook also indicated that users in the U.S., India, Brazil and Mexico were key sources of mobile growth in 2011.

In February, the social network announced it would display Sponsored Stories in the mobile News Feed, however we have still not seen a live example of this. A key point for investors will be whether Facebook can successfully monetize mobile activity as users spend more time on phones and tablets than personal computers.