First, you got it backwards. It would be the social media platforms that would want to engage in censorship (and countries in my proposal would enforce them to be open).

Second, they should be careful not to let the door hit them on their way out. They'd lose the money, E.U. gains getting rid of Facebook (and gets the chance to grow its own, open, alternative).

A social-media lose - EU win situation if there ever was one...