There are still many interesting facts being unearthed in Facebook’s IPO filing today, many of which we are dissecting for your pleasure. One of the most interesting I found was that 51% of Facebook’s 845M monthly active users are mobile users too.

That means that 425M of Facebook’s MAU’s are browsing the service on a mobile device, and that worries the company for a couple of reasons.

First, the fact that they are growing in a massive way on mobile, yet display no advertising there—a product which accounted for a huge part of its revenue in the past year. Facebook states that it expects the growth rate of mobile users to exceed the ones who access the service on computers where it does display the ads.

If the company is unable to successfully implant effective advertising into its mobile offerings, this could mean a huge hit on its revenue into the increasingly mobile future.

Second, it knows that it does not control its own mobile destiny. Because its mobile applications are built on other platforms, like Android and iOS, rather than its own, it gives up some control over how those are implemented and used.

Facebook’s comments in the risk assessment portion of its IPO filing show that it considers its application platform to be a competitor to Google’s Android and Apple’s iOS:

Developers may choose to build on other platforms, including mobile platforms controlled by third parties, rather than building on the Facebook Platform. We are continuously seeking to balance the distribution objectives of our Platform developers with our desire to provide an optimal user experience, and we may not be successful in achieving a balance that continues to attract and retain Platform developers.

Facebook says that it has taken some actions including reducing how many messages apps send users, how much money it makes from applications and more. It says that these efforts to protect user experience on the service may have alienated developers from using its platform.

Facebook says that “if we are not successful in our efforts to grow our Platform or if we are unable to build and maintain good relations with Platform developers, our user growth and user engagement and our financial results.”