If they're found to be tanking a continuous fine of 5% revenue because they're too darn profitable it won't take long for the parliament to change the regulation. With sufficient harm to the consumers it's also possible to simply shut down facebook, or at least their ability to do business in the EU which would make the market completely unprofitable as they're relying on EU advertisers. They definitely can unplug each and every server facebook has in the EU. The EP is way less captured by lobby interest than the US legislature is, doubly so by an uppity US company trying to skirt EU law.

What's more likely to happen though is the shareholders firing management because picking a fight with a bully the size of the EU isn't exactly good for the share price.