I have a personal experience with using Facebook Messenger, around three years ago, my girlfriend and I heard very clearly that someone is listening to our Messenger call, he even talked for a while before stopping as if he made a mistake for not muting, this happened the next day but this time it is a woman. Both of them sound like they are American, we are in Asia and we do not talk in English during our call. That is the main reason why we used Signal app, because of those two incidents. We never trusted end to end encryption advertisement from Facebook again after that.