I'm sure you're correct that there is a price that FB/others would not be willing to pay to stay in the EU. It's a valid concern for consumers in that region, and it's one that I suspect isn't discussed enough when privacy advocates cheer on the EU's increased requirements in this area.

I'd be very surprised if the case described in the OP was the last straw for FB in the EU though. By "change their tune" I really just meant that they will have to take another position than "your laws don't apply to us even though we do business in the EU". Exiting the EU would also be a change of tune as well!

(I don't have any numbers here, and would be interested in your thoughts, but I'd be very surprised if regulatory compliance was eating as much as 10% of FB's profit in the EU. I'd guess it's more like 1%. But that's just shooting from the hip based on my experience in a more regulated space than FB's. I'd expect FB to remain in the EU until regulatory costs made it unprofitable; it would seem hard to justify any other approach to their shareholders.)