Where mobile was once a difficult challenge for Facebook, the company has successfully conquered the platform. As of the second quarter, Facebook had nearly 1 billion users accessing its social network via its mobile app each month. It also has over 300 million users viewing and uploading photos via Instagram, and 700 million users sending each other messages and GIFs with Messenger. WhatsApp has continued to grow since Facebook acquired the mobile app, and now sports over 800 million active users. Overall, these four apps alone account for over 46 minutes of mobile activity per day, more than one-quarter of the average time spent on mobile devices. Facebook already has the ad revenue to show for its mobile dominance, but there's one big reason its strength in mobile will continue to reap rewards. View the slideshow to find out.