It might interest you to know that Popper only advocated for the right to censorship in the context of democratically established political institutions, as a last resort function of a liberal democracy with a constitutionally established rule of law, which Facebook most definitely is not. Whether or not the current US government is a better steward of censorship power is a separate question, but Popper would straightforwardly disagree with this exercise of power by private parties, imo.