What dictionary are you using that says censoring can only be done by a government? It seems like a tech employee talking point so they can pretend they aren't censoring people.
I don’t work for Facebook (or any other big tech company) and don’t plan to. My understanding is that censorship is a government activity.
Private companies can censor too. For example, your ISP, browser vendor or email provider could block websites/emails/ads from a given source without telling you.
What if the private entity is censoring at the behest of the government?
Then you have a potential first amendment violation.
Censorship is in general perfectly legal, the first amendment only protects from government action, and even then, only sometimes, depending on what’s said.