So it's not an infringement on free speech to ban the depiction of an illegal act. You can't post "Come to the beheading tomorrow!" on a wall without the cops showing up, so there's no concern that YouTube would become a beheader's paradise, either.

Porn is also already heavily regulated. Again, things that are regulated in real life, apply that to social media. NBC is not allowed to show porn due to the fact that NBC is too freely available. Game of Thrones can show all the titties it likes, however, because you have to pay to get HBO. So apply the same logic.

But also, Facebook with its several billion users is not the same as TheSabre with its few thousand. Much like in Marsh, a company town is not the same as a private home or storefront. And different types of private property are already differently regulated - if I walk into someone's house unannounced, that's trespassing or burglary, but if I walk into a store, no problem. So there's no concern about regulating Facebook differently from TheSabre, either.