I would argue that anyone who takes it upon themselves to become the central hub for most discourse (or to organize the world's information) has an implied responsibility to act as a good steward of the empire they seek to control. We have no obligation as members of the public to accept whatever controls a land-grabbing (I use this term here non-judgmentally) empire builder wishes to impose; they sought to turn us into members of their empires, so we should at least have the right to demand they behave equitably, and perhaps even beneficently.
When we use a Facebook or a Twitter, we (the public) accept the product they offer. True enough, if we find out we don't like the deal we can threaten to walk away if there is no change. But these companies don't have a responsibility, implied or otherwise, to better us, cater to any of us, or fulfill someone's vision of social good. It would be nice if they delivered on these things (of course my vision of what that means) but it is not their responsibility to do so. If I don't like that with sufficient intensity, my one valid recourse is stop using their services.
The source of the implied responsibility of beneficence is the fact that without assuming such a responsibility the acts of a corporate empire are likely to be destructive to the long term interests of the society. By (for example) controlling the social network, they leave members of the society with limited choice not to participate, and thus the implied responsibility of beneficence in exchange for that removal of choice.
Having said that, their right to exercise editorial controls on their platforms does not mean they are somehow above criticism when they elect to do so in stupid ways. As the original comment in this thread suggested their moves in regard to speech are short-sighted and detrimental to the overall health of society, especially given the pervasiveness of use of these platforms. Still, I can recognize their right, and call them out on their foolishness, at the same time. I only cross the line when I demand force to be used against them (meaning a law written to force their hands); then I would be in the wrong. They don't have a responsibility to further discourse and I don't have a responsibility to hold my tongue when they fail in this regard.