I actually don't think it would effect it long term, only short term. People would use something else.
I have been critical of Apple in the past, but given the choice of Apple or Facebook, I'd choose Apple.
And I don't even own an Apple device.
I guess I just believe that people still place the quality of their products over any particular service.
I'm not sure that's true. I suspect the absence of Facebook's apps on the iPhone would be a pretty significant blow to Apple's long-term market share.
WhatsApp has >1.5 billion users, Messenger nearly that many.
(Would it also hurt Facebook? Absolutely.)