The EU is 250m of Facebook's 2.2 billion users. They could easily drop all EU business if they needed to (the business operations within the EU is the easiest aspect for the EU regulators to target). Their profitability would drop from ~$23 billion for 2018 to $19 billion or so at worst. That'd be made-up for with one year of growth. Once Facebook's business operations in the EU are shut down, Facebook could continue to operate the user accounts and leave it to the EU to attempt to censor what their population is allowed to access online.
Watching the EU attempt to install Chinese style repression and censorship to limit what its people can access online, would be quite the comical circus.