They might have some leased servers, but afaik none of their custom built data centers are located there. Compared to abandoning a bespoke facility, packing up and moving out of a server farm is little more than a hassle.
http://www.datacenterknowledge.com/the-facebook-data-center-...
And just seizing and selling their IP would be enough to encourage facebook not to violate the law.
Everyone would love buying their software and trained models.
Kim Dotcom is a good example.
Companies having to follow the law of where they provide their services is a far deal less prohibitive.
And, frankly said, I'd love it if Google, Facebook etc would leave the European market — as local competitors would appear very quickly, and, due to the huge size of the European market — would actually become viable competitors to the American companies even in the international market.
Germany has seized assets of corporations (for example, they did seize an airplane, to force the airline to pay back the cost of a ticket to a traveller whose flight was cancelled) before to enforce regulations.
Facebook will definitely want to avoid their hard- and software being seized and sold off to pay for a potential fine.