> The Free Speech Clause of the First Amendment constrains governmental actors and protects private actors. To draw the line between governmental and private, this Court applies what is known as the state-action doctrine. Under that doctrine, as relevant here, a private entity may be considered a state actor when it exercises a function “traditionally exclusively reserved to the State.” Jackson v. Metropolitan Edison Co., 419 U. S. 345, 352 (1974). [0]

[0] https://www.supremecourt.gov/opinions/18pdf/17-1702_h315.pdf