A S-1 initial public offering document filed with the SEC on Monday revealed that Facebook has surpassed 900 million users, 500 million of which are mobile users. We’re not surprised by the staggering amount of mobile users: Facebook’s mobile app is available on nearly every mobile platform. Plus it’s easier to use a mobile phone or tablet instead logging into a computer to use the site.

As Facebook’s user base continues to grow, its operation expenses are on the rise as well. The company’s spending doubled during the past year and its net income dropped 12 percent from $233 million during the year ago quarter to $205 million during the first quarter of this year. The company’s revenue of $1.058 billion was up 45% year-over-year but down six percent from Q4 2011.

The S-1 filing also revealed that, if Facebook’s planned $1 billion acquisition of Instagram should fall through, the company will have to fork over a $200 million termination fee to Instagram.

[via: Reuters, Business Insider]