First amendment, and rest of constitution, limits power of government. In case of first amendment, it prohibits Congress from “…abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble…”
This can/should be extended to Federal agencies, and federally funded entities. State constitutions have similar protections, limiting state and local government’s attempts to abridge free speech. Laws can be passed requiring federal contractors to extend certain rights to their employees or customers, but that not required by 1st amendment.
So is this essay suggesting a law should be passed prohibiting Social Media companies from regulating their content? That doesn’t sound very libertarian.
Rather individuals and companies can have speech codes, limitations on free assembly or even religious tests (except for employment). They can be enforced within the home, businesses, or with whom they chose to interact.
Facebook and Twitter censorship is a concern. And if mismanaged should cost them business. However, it would be ludicrous to suggest that a private business cannot set its own user agreement; and first amendment limited not just Congress's power, but everyone (who used public infrastructure)’s power.
If a theater relied on a public parking lot, would it need to allow nudists, or fowl mouthed “sailors” to parade naked and swearing through aisle of g-rated movies, interrupting the feature length cartoon? Does a bar that uses public airways for its antenna tv thereby forfeit its right to refuse entry to a protesting assembly of teetotalers, or a previously disruptive patron?