I suspect the real real answer is that Google would summon an enormous shitstorm of antitrust action down on their heads if they even thought about kicking their biggest domestic competitor in the ad space out of the app store.
Can Facebook violate any TOS and not get kicked off because of antitrust?
They could justify it to a court, because "Facebook incubated the insurrection" or "Facebook broke our rules" not because "we don't like enabling our competitors".
Apple disabled both Facebook and Google's iOS enterprise certificates because they were using them to collect data from external participants in violation of the ToS, although that wasn't the main app. And the access was restored once they took down the offending apps.
If the violations were serious enough, I think Apple/Google would consider some kind of temporary shutdown for the Facebook app if they thought they could weather the PR storm.
Apple has less to fear from antitrust than Google in general, and particularly from taking hostile actions towards Facebook.
On the other hand, Facebook is not really a competitor to Apple, while Google very much is, so Apple has an incentive not to carry out an action that would strengthen Google.
For the purposes of making a decision like this, justifiability in court seems like a relatively remote worry. The shitstorm itself, I'm guessing, would be the more immediate concern. Few people in a position to make a decision like this would have neutral feelings about the prospect of a swarm of federal investigators giving them the white collar equivalent of a body cavity examination in order to suss out whether their intentions were pure.
That's true. At least in the U.S. anti-trust laws require you to show that the company engaged in certain actions with the _intent_ of creating a monopoly.
The real answer is that neither Google nor Apple should have been allowed to create walled garden unregulated marketplaces that we're forced to use if we don't want our devices hacked.
There is only one logical conclusion and that is that Apple and Google will eventually boot Facebook off their platforms due to increasing amounts of risk associated with mobs and things that allow them to form.
