Facebook (and Whatsapp, and Instagram) has a truckload of users. I think people would get really mad if they were forced to close shop in Europe. (Not sure that rage would lead anywhere, considering how the EU works...)
But it seems like they wouldn’t stay mad for long and there wouldn’t be any significant economic impact of them just switching to other services.
This is not the same as say if Apple suddenly stopped selling iPhones and Macs or Microsoft stopped providing Azure services.
The end-users, they still want to use Facebook, Instagram, WhatsApp, etc. And they will be quiet vocal if those things stop working.
The fact of the matter is that people and businesses rely on Facebook and they won’t have the time or patience for an explanation beyond them from the governments out to get FB.