Reveal's, diagram inside Facebook London office seems to be terrifying Apple.

For years, Apple has dominated the $45 billion app business with its App Store. Android has always been the second-best place for apps.

Facebook, however, has a plan that could change all that. This hastily scrawled diagram on a whiteboard in its London office represents that plan, and it ought to scare the folks at Apple who work to maintain the primacy of iOS among app developers. (We explain the diagram below.)

iPhone and iPad users are generally more lucrative than Android people for apps, in terms of download fees and in-app payments. So the best apps are developed first for Apple's iOS mobile operating system. Only if they are successful do companies produce an Android version, often months or years later. Even then, Android apps tend to be copies of the original iOS app, with all the flaws and compromises that implies.

The dominance of iOS creates a bizarre distortion in the app market: Apple only has a 12% share of mobile users; 80% of users are on Android. Yet the 4-to-1 majority is treated as second class in the app world. It can actually be difficult to hire Android developers because staff only want to work on iOS. And you can be hugely successful as an app creator, even if the vast majority of phone users have no contact with your product.

At Facebook in Europe, however, executives think that Apple's iOS dominance might be about to weaken.

We spoke to Facebook's Europe, Middle East, and Africa platform director Julien Codorniou recently and were surprised when he told us that the trend he was seeing favored Android. Revenue generation on Android is catching up to iOS, and more developers - particularly for game developers - are going Android-first, especially in Europe.

Facebook is hoping to take advantage of this via its Parse app development platform, which Codorniou believes virtually erases the two-step iOS/Android development process, letting companies release new apps on both platforms simultaneously.

Erasing a key selling point for the iPhone.

That would actually be good for Apple in the sense that a robust, growing app market only increases Apple's App Store revenues. But it would be also bad for Apple because if apps are released at the same time on Android it erases one of the key selling points of the iPhone: That users who want the cool new apps must be Apple users in order to get them.