To answer your first question, the constitutional response would be "it doesn't". Legally, I'm unaware of any edge cases, except potentially some net neutrality related things. (I'm not very certain about NN.)

My point, however, is that this is neither a legal nor constitutional obligation, but a social and voluntary obligation to a free society.

Agreed. Just wanted to see if there was some precedent I was missing :)

Comcast is few-to-many model, Facebook is many-to-many.

A few content producers create the content that Comcast could potentially carry. Comcast selects what they want to provide to their many customers. Comcast has never marketed themselves as a place where anyone can have their content provided to subscribers. Facebook has, in fact that is the essence of what they do.