If someone hasn't read your message in a month that means a different thing than no response for a month. Some would assume you said the wrong thing, etc. Also remember Facebook is software- which can fail. Maybe this is also a way to help reduce message delivery failure- with email you might get a message bounce back if the send failed. If a Facebook message failed to deliver due to bugs, hardware failure, etc how would a user know? If they don't know they probably won't complain to the FB message API developer/manager who has a "can't reproduce", "intermittent", "sometimes occurs" bug.