I think you're right. My suspicion is that if FB/TWTR go totally ban-happy, the social discussion will evolve into a legal one and move towards overt regulation a la utilities.
EDIT: IANAL, but if these platforms start favoring one political group over others, it seems potentially illegal, as it could be considered in-kind political donations.
When people make the argument of-wrt suppression or censorship of speech "Legally no but socially/in the court of public opinion yes", I often want to fire back "But...are not our laws a distilled and concentrated output of what we value in social publics? If we start allowing exemptions and exceptions to that value system to get an outcome that takes away liberties in social publics, are we unwittingly creating the 'case' to erode those values in (the grander) legal establishment-given enough time?"
(This might be a long winded argument of 'slipper slope', unsure. Someone call me out on this if so)
Maybe it's a question of ethics versus law, or perhaps ethics in the context of buffers between person/individual and state/government?