Facebook, the social networking giant, is readying an open mobile operating system that could eventually compete with Apple and Google for app revenues. The company announced today that it is partnering with mobile carriers to offer carrier billing on Facebook-based transactions. In turn, this could lead to an app store ecosystem for everyone, according to TNW.

Announced at the Mobile World Congress, Facebook is testing a system, code-named Open Graph, which allows users to purchase and download Web-based apps from any mobile browser. For would-be developers, Facebook is making it “exceptionally easy,” to come on board. All that is required from them is to implement the company’s new Pay Dialog into their app. From there, Facebook and the carriers will do everything else.

This news is significant in that Facebook has 425 million monthly users, most of whom use a mobile device. With Open Graph, these users could take their apps with them, regardless of OS.

For example, Android-based customers could move to an iPhone, and vice versa without losing their apps. This barrier to entry is one of the reasons many mobile customers continue using one mobile OS over another, upgrade to upgrade.

In November, rumors suggested a Facebook phone would arrive this year or in early 2013. Code-named Buffy, the first Facebook Phone would come from HTC or Samsung, according to our original report. It is unknown whether today’s announcement confirms a Facebook phone, or rather suggests the phone is no longer a possibility.

A Facebook mobile OS would certainly have the potential of changing the entire industry. Still, whether users suddenly begin switching exclusively to Web-based apps remains to be seen.

For one, an open source OS would present the same security problems associated with Google’s Android OS. Second, Facebook apps would have to be as good as, or better than, those offered through Apple’s closed App Store.

Still, there are already millions of Facebook users who enjoy the service’s Web-based games on a desktop. Converting just a few of these to mobile apps would be huge.

Would you consider buying Facebook apps as replacements for those being offered by Apple? We’d love to hear your thoughts.