Facebook’s Mobile Monthly Active Users Grew 21% Over Past Four Months

Earlier analyst estimates from December had pegged the monthly active users of Facebook’s mobile apps at around 300 million per month. This number includes smartphone apps, like those for the Android and iPhone, but also apps that run on BlackBerry, Nokia, and feature phones. At the time, that number equated to roughly 40% of the company’s overall user base.

Facebook hadn’t revealed an official number for monthly active users on mobile since September, however, which was then at 350 million users for both mobile apps and mobile web combined. Today, thanks the Facebook IPO filing, we have an update to the official numbers: there are now 425 mobile monthly active users as of December 2011, out of Facebook’s total 825 million users.

Facebook defines a monthly active user (MAU) as a user who accessed Facebook via a mobile app or via mobile-optimized versions of the Facebook website (e.g., m.facebook.com), whether on a mobile phone or tablet device like the iPad, during the period of measurement.

The company said that mobile usage of Facebook increased in 2011, including in major developed markets like the United States where smartphone penetration has been rapidly growing. But the mobile MAU has also been driven by other product enhancements, including Facebook’s acquisition of Snaptu in April 2011 and the launch of the long-awaited Facebook iPad app in October.

However, despite the large numbers of mobile users for the social network, Facebook doesn’t currently display ads through either its mobile apps or websites. That’s a large, untapped market for Facebook’s advertising efforts, since, in many cases, users only engage with Facebook via mobile – not on the desktop-sized web. Not surprisingly, Facebook didn’t assign a number to its “mobile only” demographic. When it comes to how many Facebook users only interact with the social network via apps or the mobile website, it’s still going to be anybody’s guess.

Facebook also notes that its revenue may be negatively affected by the fact that it doesn’t currently monetize its mobile user base via ads. This will continue “unless and until” it rolls out either ads or sponsored stories (promoted stories from advertisers) to mobile devices. The global mobile advertising market was $1.5 billion in 2010 and is expected to grow at a 64% compound annual rate to $17.6 billion in 2015, Facebook notes in the filing. With that number in mind, Facebook says it believes in its potential future monetization opportunities. (No kidding).