Facebook (and Youtube et al) have chosen not to be content-agnostic distribution platforms (unlike the postal service and SMS). They perform a variety of editorial functions to curate content and provide recommendations with the objective of maximizing engagement and hence advertising impressions that they can sell. This is popularly known as "the algorithm".
Given that these platforms have chosen to selectively display content for profit, many people believe that they should have some sort of responsibility for the editorial decisions they make.