If FB gives up on the EU market, then they've opened a massive weak spot for themselves. Social networks benefit from the more connections between people.
If FB leave the EU, then some EU company can copy the software (we know what kind of features people want), and this company will be able to operate in EU and USA, but FB will not be able to operate in EU, giving this EU company a massive benefit and safe harbor.
If FB leave the EU, then some EU company can copy the software (we know what kind of features people want), and this company will be able to operate in EU and USA, but FB will not be able to operate in EU, giving this EU company a massive benefit and safe harbor.