The iPhone update doesn't even immediately forbid tracking, it just requests apps to ask for permission. That simple change pushed Facebook to buy newspaper, radio and TV ads to try to get people to reject the update. Think about it: they feel threatened by user awareness.
The dialog pops up and you give your consent for the FB app to track whatever it wants, because you're fine with that.
Other users similarly are allowed to make their own choice as to whether they accept that trade off.
I don't think this is an entirely fair argument unless it is exceedingly clear what you are opting in to. The fact that FB (and others) are so resistant to being more transparent about this is in itself informative.
Separately, opt in implies informed consent. Most users have no clue what data Facebook is capturing. It's not informed.
Isn't it opt-in by using their platform? Or is your argument that targeted ads are not common knowledge to Facebook users? I likely wouldn't use Facebook or Google etc. if I had to pay cash; fine with paying with personal data; but for people who aren't fine with that, they shouldn't use it.
I realize that any attempt to discuss this is inviting downvotes, but I just ask for a quick comment if you do choose to downvote. I try to be very open minded.