In that case, they should probably not use the system while trying to stay anon. The system will not work in their favor, and Facebook is definitely part of that system.
anybody could be part of the system knowingly or unknowingly. so the best is to always stay anon and trust nobody. all of the system should be designed bullet proof anonymity just like apple does, even if apple wants to can't read messages.
That's a very different thing. End-to-end encryption stops Apple from reading messages on its own initiative, but if you discover that someone with a particular phone number is sending defamatory iMessages, Apple can still tell you who that person is. (Even the idea of a phone number being anonymous sounds kinda silly, even though it's no structurally different than a Facebook user ID.)