Here's the relevant excerpt from the IPO filing amendment:
Online display advertising typically includes banner ads, interstitials, video ads, and rich media ads that aim to reach large numbers of consumers within a particular audience segment. Display advertisers run impression-based campaigns on Facebook in order to reach our large user base and because of the amount of time that users spend with us. From January 2011 through January 2012, Facebook.com has been the number one online property accessed through personal computers worldwide as measured by total minutes spent and total page views, according to a comScore Media Metrix report dated February 2012. On average, users in the aggregate spent more than 10.5 billion minutes per day on Facebook on personal computers during January 2012. Aggregate minutes per day increased 57% and average minutes per user per day increased 14% during January 2012 as compared to January 2011. Display advertisers also use Facebook in order to more precisely reach their target audiences among our users and to leverage social context and our social distribution channels to increase engagement
Given that Facebook has 845 million monthly active users as of December 2011, this means each user is spending an average of 12 minutes and 26 seconds on the social network. It makes you wonder how much times those millions of fake accounts are spending. Unfortunately, Facebook did not include information about time spent by mobile users on the site.
When the company updated its IPO filing earlier this month though it did say that it ended 2011 with 432 million monthly active mobile users. The company also revealed for the first time the number of monthly users who only access the site from a mobile device: 58 million, or 13.4 percent.