Definitely not true. Facebook literally censors private conversations. You simply can't send certain text strings to your friends. That is far more dangerous than relying on a third party that claims to be protecting your privacy. Especially since all signs point to them being honest.
I don't know about WhatsApp (but I also didn't mention WhatsApp), but go to FB Messenger right now, open up a conversation with yourself, and try to send a message containing the string "thedonald.win". You'll get an error message saying "Couldn't send", with no further explanation. The list of banned strings used to be longer, but they've unbanned a lot of them since the election ended.
To be clear, this is in private conversations. Not just posting publicly on Facebook or w/e.
Definitely not true. Facebook literally censors private conversations. You simply can't send certain text strings to your friends. That is far more dangerous than relying on a third party that claims to be protecting your privacy. Especially since all signs point to them being honest.