Like all social contracts, it's subtle. Facebook is the "meeting place" and final location of sharing the content. Similar to an assualt happening in a park at

Like all social contracts, it's subtle. Facebook is the "meeting place" and final location of sharing the content. Similar to an assualt happening in a park at night, you could look at the trainers the perpetrator wore, the mode of public transportation they took, etc, or the council could install lights in the park as they control the space (not a perfect metaphor). Responsibility is the burden of those with the most means to enact change. There's no formal chain of logic for deciding where that burden lays. Your example presents every actor in the chain as equal options but depending on your political (philosophical) leaning you will want different actors to burden more or less of the responsibility. (control from the source or control from the event, or anywhere inbetween).