It's reasonable to have moderation rules that prevent death and harm...but...I'm not actually asking about moderation. I'm asking about what, if any, obligations "social media" companies have to their users.

Carriers are legally forbidden from moderating on the basis of content. AT&T isn't allowed to eavesdrop on you planning to attend a rally somewhere and terminate your cell phone service for violation of contract. The reasoning is that AT&T isn't and shouldn't be responsible for your content: the responsible party is you. AT&T just provides the technological extension of your voice to reach farther, but otherwise all the same rules apply. They're like the air: they carry your information.

Media producers, on the other hand, typically have sole discretion over their content. If their CEO hates cats, they can avoid publishing anything at all that talks about cats in a positive light. If they interview people after a concert and 5% of them say they hated it, 95% say they loved it, and the producer publishes only the haters, that's again totally and completely legal. They have no obligation to give voice to the 95%. Of course, people might not like the media producers' editorial discretion, so they might choose to avoid it if they have a credible alternative. But the standard legal setting allows them can filter everyone's voice to any degree they want (so long as they don't actually evoke violence, or knowingly defame someone to such an extent as to trigger libel laws--these things aren't there to facilitate hearing voices but rather to curtail the most egregious harms).

So, what is Twitter is a really important question in this context. To the extent that it is like a carrier, it has obligations to not interfere with what people say (beyond some minimal "no crying fire in a crowded theater" extent). To the extent that it is actually media, it can do whatever it wants. (If we follow existing rules. Of course, rules can be changed.)

My impression is that Cory's piece accepts that social media has essential carrier-like characteristics. And my impression of your response is that you deny that to such an extent that it's hard to even recognize that you comprehend what Cory is talking about. But it's all implicit, and I'm trying to make it explicit, because I don't think you actually think that Twitter is more like the Daily Sun than it is like Verizon.