You mean that in a newspaper it's not the newspaper saying it, but only the "contributor" (journalist) writing it? Sure, sometimes it happens (i.e. op-eds) and the reader is usually explicitly warned. AFAIK, the newspaper is still responsible for what it publish.
Both the newspaper and Facebook (& C.) are not neutral entities - both businesses are built on monetizing directly the "contributors" contents (including matching them with ads) - plus both control which are given priority and visibility and which not - using their "algorithms" (it doesn't matter if it is a director of an AI trained as you like).
On the other side a printing press company doesn't monetize contents but is just paid to allow their display, and makes money only from the number and quality of the display, not the value of the contents - and still I believe can be liable if it prints illegal material knowingly.