They have a very easy choice actually. Either abide by the rules if they want to service EU citizens, or GTFO. If they want to give up a couple hundred million users and hand them over to the GDPR-compliant competition, that's their choice. It would be monumentally stupid, though. FB needs EU users a lot more than EU users need FB, but they're trying very hard to spin the press to make it appear the other way around.
It's like when McDonalds or a similar tax-dodging company threatens to leave a country if they face being forced to actually pay the taxes they owe. Leave millions and millions in profit right there on the table for their competitors to grab? Yeah right, ain't gonna happen.