Are companies that provide one-to-one communications services, like cell phone companies, complicit in drug dealing, insider trading, and all the other nefarious acts that are no doubt organized using their networks? I think most people would say they're not. Meanwhile, one-to-many communications services, like broadcast TV, by their nature require editorial decisions. If you're only broadcasting one stream someone has to decide what the content is, and thereby becomes responsible for that content.

Social media is many-to-many. Quite a few people seem to have decided this imposes the same sort of centralized responsibility that exists with one-to-many communications, but this is not at all obvious to me. On social networks there is no single stream of information, thus there is no necessity for editorial decisions, thus the mechanism by which one-to-many broadcasters become responsible for the content they carry is not present.

Recommendation systems complicate this somewhat, admittedly. If platform owners are putting their fingers on the scale (algorithmically or otherwise) to determine what gets exposure, some of the responsibility comes back. However, in the simple case of e.g. Twitter showing you content from people you've explicitly chosen to follow, I don't see how they've got any more responsibility than exists in the one-to-one scenario.