It's their platform, their software, they have to bear some responsibility for the content. If they aren't, not form of media is. It's like a newspaper saying we aren't responsible for any content that appears in our paper that we didn't produce ourselves. We only supply the paper and distribute it.
Analogies with traditional media just don't capture the broad reach and accessibility of the internet.
It isn't just what's posted that's a problem. It's the algorithms that drive readers to more extremist propaganda, and then feed them a steady diet of material - eg, of anti-Muslim and/or white supremacist hate propaganda, all exaggeration, fabrication, memes lacking in context or sources - that are very frightening to uninformed people who share it widely to warn others, make others fearful, angry, anxious.
Those algorithms are the responsibility of the platform. They make money from click bait.
That's where we should be directing concerns, I think.