The Facebook app on Android exists only because Google allows it. It is distributed through the Play Store and uses Google services for everything from in-app payments to push notifications. If Google wanted to, it could block the app from the Play Store, which would limit the app’s reach catastrophically.
It isn’t like Google and Facebook are strangers to conflicts; Google has had problems with the Facebook app before. Remember when Facebook set the app to update using files outside of the Play Store? That didn’t go well with Google. Though Facebook brings Google business, it’s also competition and it’s not too far-fetched to think that Facebook could anger Google enough to get its removed app from the Play Store.
Naturally, Facebook has plans in place for such a situation. Though the company doesn’t expect it to happen anytime soon, it’s important to plan should things go wrong. Facebook would then be tasked with getting users to download the app outside of the biggest app store on Android, which won’t be easy (though it seems people are willing to do so to get their Facebook fix).
Another hurdle is replacing important Google services, like payments and notifications. These services can be recreated by Facebook, but the issue would be cost. Facebook could let other companies use these services in countries where Google services are either not available or not popular. Another option is to work with carriers on pre-installing the app on devices, which carriers already do through the Play Store.
While it seems a bit crazy to be planning for such an occurrence, it makes sense for a company that can stand to lose a lot by losing the grace of Google. Hopefully Facebook won’t do anything silly enough to do that.