This clearly implicates that Facebook has, almost without reserve, the ability to read messages from user to user (even though it might limited to messages to oneself), and exposes that ability to its employees.
You can view and search your message history on Facebook, so there is not really any question that they have and can read all messages. I never explicitly checked it but at least I never noticed any gaps, i.e. missing messages because I sent them using the Messenger app.
I believe to remember that Messenger is supposed to use Axolotl Ratchet for end-to-end encryption but that is hard to reconcile with the availability of your message history on facebook.com. So maybe it's - quote - end-to-end - unquote - between the phone and the server?
I never really thought about that. Or is it just not available or disabled in my Windows Phone version of the app?