In yesterday’s Facebook IPO filing, the company revealed many details about its user base, including its mobile users. In December alone, Facebook had more than 425 million monthly active mobile users – that’s a little over half of its user base.

The social network defines a monthly active user as anyone that accessed Facebook through a mobile app – including Android, BlackBerry, and iPhone apps – or through the mobile version of its site,

Contributing to the increase in mobile usage was the company’s acquisition of Snaptu – which launched a Facebook mobile app for a variety of phones – in April and the launch of the Facebook iPad app in October.

What’s interesting is that while Facebook has been experimenting with a variety of ad formats on its website, the company currently doesn’t feature any advertising in its mobile apps.

However, Facebook did reveal in its filing that it’s aware of this untapped market and alluded to future plans: “we believe that we may have potential future monetization opportunities such as the inclusion of Sponsored Stories in users’ mobile News Feeds.” This shouldn’t surprise users as the social network began testing Sponsored Stories inside News Feeds on its web version last month.

We expect Facebook’s mobile usage to continue growing. One thing to keep in mind is that interaction with Facebook Pages is still somewhat limited on a mobile device. Meaning, that if you utilize apps or various tabs regularly – for example, when hosting a contest – your fans might not be able to access the necessary tabs. Make sure you balance your Page with mobile-friendly content so mobile users aren’t missing out on anything important.

[Via: VentureBeat]