[nicknyc]

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.

[loceng]

If it's important you'll find another way to contact them.

The logic behind using it to show if they fail or not doesn't work. The system that would identify if a message was sent would be tied into the messaging system; They should have this if it's possible, though they don't need to make the results known.