> If everything is censorship, seems to me like nothing is.
I will give a definition that counters this.
Censorship is not a binary thing. It is a spectrum.
Specially, the definition of censorship that I would use, would be "Any action at all, that chills someone else's speech. But the more power and effective the action is, the more that it is a problem".
So, government censorship is a problem not because it is the government doing it specifically, but it is a problem because the government has a lot of power.
To give another example, if a very power criminal organization was threatening to kill anyone who said bad things about it, and these threats were real and worked to chill people speech, then I would consider this to be possibly as bad as the government doing it.
To address your other example, the main question that I would ask is how powerful is the action. You refusing to listen to someone doesn't very effectively chill speech.
But actions such as very large and powerful companies censoring things is much more effective than a teacher censoring things, and thus a larger problem, although not as big of a problem as if someone threatened people, en mass, with violence.
To address the Facebook example, the question that I would ask is "How powerful is facebook, and how effectively are they able to wield that power to chill any form of speech? Are they very effective at controlling speech or not very effective at it?"