GDPR won't dent Facebook in the least. They'll bend where they have to, and no further. They'll pay occasional fines and that won't matter either.
If you take it to an extreme and the only thing they can do is show non-personalized advertising and do zero tracking, they'll still print billions in profit in the EU market. That's what owning most of Europe does for you. Their gross profit margin globally is just under ~87%. Cut that in half and they still have a very profitable business with margins comparable to Google.
Facebook could run entirely untargeted advertising on their platform globally and still yield enviable profit margins. Their platform is extremely tightly run on costs compared to most tech giants.
People talking about doom on this, aren't discussing facts or using logic, they're projecting a revenge fantasy out of emotion. It's driven by a desire to punish Facebook.