And you just trust Facebook to somehow really encrypt everything in this black-box? I think this is naive as Facebook has always lied to us and proved it isn't

And you just trust Facebook to somehow really encrypt everything in this black-box? I think this is naive as Facebook has always lied to us and proved it isn't a trustworthy entity. WA used to steal your contacts by default to improve Facebook's data set and probably still does. The Telegram guy however left his country because he didn't want to cooperate with such entities (like the Kremlin which for me is kind of another organization that seeks to harm your privacy among other goals and is therefor comparable to FB and other spy-companies).