Plenty of companies made money on Windows without directly paying Microsoft. Google gets some revenue from Android OEMs licensing the rights to distribute Gapps. If Facebook moves Android phone sales, Google makes money. Not nearly as much as MS did per unit, but Android is also installed on way more devices today than Windows ever could have hoped to be.
Google does advertise on Facebook. There's no way for them to put AdWords or AdSense ads on Facebook without their cooperation, and if you're suggesting they do it through Android, they can have a blast trying to sell Android phones when Facebook pulls themselves from them.
And just what will those users buy? The Samsung-S7 (and equivalents) will buy a similarly priced iPhone. The rest have no alternative. BlackBerry is down and out. Windows Phone essentially is too. The only remaining players are iOS (at 10% of the market and 90% of the profit) and Android (with the rest).
That's my point. Google would never use their control of Android to serve their own AdWords or AdSense ads on Facebook on mobile, because the loss of Facebook would push the Android segment that makes them money (the higher end segment) towards iOS
