Facebook Home is coming. It’s a unique threat to Google’s mastery of Android that Google can blame nobody but itself for. It’s the unique nature of the threat — both Home’s technical foundation, and the nature of Facebook’s rivalry with Google — that I believe makes Home the first real test of Google’s “open” mantra regarding Android.

There have been many challenges to date, but most of them involve “forks” of the Android system: where another company builds their own system on top of a previous Android release. Amazon’s Kindle Fire, Barnes and Noble’s nook, and Samsung’s rumored / assumed proprietary fork are all notable examples of Android derivatives. None of these to date have looked poised to take Android away from Mountain View anytime soon. Samsung, with its market share momentum and marketing prowess, could stand to be a significant threat, but for now its flagship products still run Google’s “stock” Android, more or less.

Horace Dediu put it plainly:

Facebook Home can only reside on Android because only Google was daft enough to allow it.

I’d add “for now.” Now we get to see how true to its ethos Google and Android stay. How long will Home remain on the Google Play store? How often, and how mysteriously, will it have “compatibility issues” with new releases? How long before launchers in general start to get buried under convenient categorization?

At the least, I expect an increased emphasis from Google on the virtues of “stock” Android, and an increased push to make that consistent for consumers. This is already underway on both OEM and developer fronts, but Facebook’s lurking presence will force the issue that much harder.

Home’s to-be-determined success could also force Apple’s hand. Such a product is neither technically nor legally feasible on iOS at the moment, and Facebook’s integration into iOS 6, while powerful, is much less than Facebook Home provides on Android, and thus presumably much less than Facebook wants on iOS. As I said last week, if too many consumers start considering Facebook Home a deal breaker, Apple may need to make some moves of its own. How many consumers that is, and whether they’ll in fact get on board, remains to be seen. All of this just underscores what’s at stake for everyone — Facebook most of all. With just one announcement, Facebook has made itself a strategic stakeholder in the mobile landscape. It’s no longer just a website and an app.

Google knew what it was doing when it made and marketed Android as an “open” system. It surely anticipated forks by handset makers as a manageable risk as long as Google kept advancing the system. But I wonder if it expected something like Facebook Home: an inside-out heist, made by a company after the same exact user data and advertisers Google is after. How it chooses to respond in the near future should give us an answer.