In the article you linked, Cyber Promotions v. America Online seems far more relevant given that it pertains to an individual's right to use an online, non-state-owned platform as a host for arbitrary speech.

In the decision[0], they cite three distinct tests used in previous cases to determine if the company acted as the state by asking if its activities are "state action" (i.e. meaning that removal of speech constitutes a violation of first amendment rights)

1. Is Facebook exercising the exclusive prerogative of the state in its activities?

2. Is Facebook acting with the help of or in concert with a state official?

3. Did the government insinuate itself into a position of interdependence with Facebook to assert itself as a joint participant?

All of these hold for the Marsh v. Alabama ruling, but the argument that these tests pass for Facebook's daily business activities is less convincing.

[0] https://scholarship.law.berkeley.edu/cgi/viewcontent.cgi?art...