Problem is that we are at a point at which these 'private platforms' are in a position to do serious damage to public speech primarily due to network effects. They dont become important venues of speech until they are a dominant portion (if not outright monopoly) of communication medium. This is structural & will not be resolved on its own unless some other non-profit seeking entity (Govt) enforces it. As it is we are losing the 'money is speech' battle because of 'Citizen's United' (google it), This left unchecked will just make it hopeless.