Regardless of whether this is a good idea or not, how will it possibly be enforced against a multinational giant like Facebook, with private source code and machines to store data all over the world?
Generally by leveraging the fact that Facebook probably wants to remain on reasonable terms with most governments. Modulo appeals etc, once they've finally lost (provided that's the final decision), they'll probably just comply.
If they refuse, the government can seize any assets Facebook might hold in Belgium, possibly other EU countries, they could block or fine Belgian companies and individuals that do business with Facebook and such things, and escalate all the way to issuing warrants for Facebook executives' arrests, which with the European Arrest Warrant could be effectuated across the EU.
