As people usually say here: It's not censorship unless a government does it.

Not really. Censorship is generally used in a general sense, in which a private party can be the one doing the censoring. It doesn't purely apply to the first amendment in the US constitution.

And with sites like Facebook, Twitter, YouTube and others with a large share of the market, it can certainly seem like censorship given how much being blocked or banned hurts your visibility.