The First Amendment protects us from government censorship. But private companies, like Facebook or Twitter, can set their own standards. For example, a restaurant can say “no shirt, no shoes, no service.” Similarly, a website can have rules that say no harassment, no hate speech, no porn – as long as they apply to all users equally.

Supreme Court of the United States: Manhattan Community Access Corp. v. Halleck

United States District Court, S.D. New York: Knight First Amendment Inst. v. Trump

Congressional Research Service: Free Speech and the Regulation of Social Media Content