By Justin Sherin

In documents filed with the Securities & Exchange Commission (SEC) in advance of its initial public offering, Facebook revealed that it now has 901 million monthly active users, compared with 680 million just a year ago. The documents also state that over 500 million users engage with the site on mobile devices.

By any measure, this is an astonishing rate of growth. To put it in perspective, the current population of the United States is about 311,600,000 people. For now, the world really does live on Facebook, making the site an essential component of any social media marketing campaign. However, Facebook's SEC filing was compiled before the switch to Timeline, which re-opened questions about user privacy and changed how advertisers interact with the site. [links] We'll let you know if the change affects Facebook habits.