I'll guess that Zuckerberg's definition of "private" is not at all the same as yours or mine. Personally I'd define "private messaging" to mean a way of talking

I'll guess that Zuckerberg's definition of "private" is not at all the same as yours or mine. Personally I'd define "private messaging" to mean a way of talking with other people (via tech, obvs) such that nobody else knows anything about it - neither content nor metadata/network. But there's another possible definition, much looser, along the lines of "Having a conversation with somebody else" with no significant restrictions on eavesdropping or metadata collection - something closer to having a "private" conversation in an airport lounge: it's not intended to be heard by anyone other than the participants, but who knows who's listening in.