But, for the record, they regularly get sued. I think they are being seriously sued in India for defamation at the moment, for example.
Why do you think facebook is ending fact-checkers now? Editors are hired by facebook, facebook is the publisher. If facebook publishes "fact" and people get harmed as result, Facebook gets sued to bankruptcy. There is no protection from government anymore!
What a nice reality would it be where Facbook could be actually sued to bankruptcy for whatever reason, let alone such minor one. Sadly it's not our reality.