Apple explicitly barred Facebook from rewarding its users for opting in. Obviously this is done with the color of protecting its users (from, say, Facebook gating the entire functionality of its app behind the permission check) but it's still a restraint on Facebook's freedom of trade.
People are opting out because Apple have explicitly barred Facebook from offering them anything as a reward for opting in (other than better targeted adverts, which most don't see as a positive). Obviously if they didn't do this Facebook would use dark patterns (such as crudely gating the entire functionality of the app behind the opt-in) but it's disingenuous to say that this is a free exchange when one party is prevented from bringing anything to the table.