In its Q2 earnings report, Facebook revealed that it now has 399 million mobile-only monthly active users (MAUs). My math isn't that weak, so I calculated that to be roughly 30% of Facebook's total 1.3 billion user base. What does this mean exactly? Well, there are 399 million people from across the globe who log in to Facebook over a period of one month only using mobile devices. However, more than 1 billion users sign in to Facebook using mobile devices in a month.