Giving companies 1st amendment rights was ridiculous, but then giving them protections beyond the 1st amendment that no normal person gets was giving them too much power to control speech in society.
I don't see any easy answer here. I'm as uncomfortable with the amount of influence companies like Facebook yield as anyone but most of the "easy" solutions would, in my opinion, make the situation far worse.
The "they can't do it at scale" argument applies to ICS, but why shouldn't users be subject to liability when they echo something?
We live in an approximation of the infinite monkeys with typewriters. It's relatively easy to find someone judgement-proof who has written pretty much anything that you'd like. Folks with huge audiences can use that to be as defamatory as they'd like without risking liability.