I guess Apple will have to weigh whether they'll want to tolerate the Fb app on iOS, but more probably they have a deal with Fb (else they would've kicked them out already). Considering Fb is said to be the main portal for "the web" as a Fb user might see it (which I find still very hard to believe for a number of reasons), if the Fb app (or worse, WhatsApp) isn't on iOS, with Safari and FF already blocking most tracking on facebook.com, Fb might block Apple devices altogether, and Apple might or might not stand to loose in device sales, depending on the intersection of Apple's with Fb's target demographic. If it were only for Spotify (which is a direct competitor to Apple Music), they'll have to weigh whether them kicking Spotify out will be seen as anti-competitive vs having a good excuse for eliminating a competitor on their own platform. But maybe my speculations are way off, and Apple will just tell Fb to fix their shit.