"It's not censorship if we can still whisper in the back alleys."

So if Facebook isn't ubiquitous and omnipotent they cannot be considered a censor? You're deliberately applying an impossible requirement that you made up. By this requirement there is nobody that can censor, "China doesn't censor because you can still talk about what you want in America." It's absurd.

FB censors private messages between two people on Facebook Messenger. That's the claim. I'd say it's pretty well backed up with proof and facts in this thread.

Censoring is a verb. They censor (remove content they deem objectionable) communication on their platforms. That is the claim. You’re setting up some kind of straw man here saying they aren’t “a censor” because other platforms exist. By your definition, no true censor exists because someone could always communicate directly 1:1 with someone.