AOL did this back in the 90's for any mail sent within the confines of its walled garden. You could also unsend a message.
At the time, I believe the purpose was to engender a sense of trust in "the new medium of email." Obviously that's not the purpose of the implementation on FB.
So, what is the purpose?
Maybe it's the extremely late hour, but my cynical feeling is that this seems really a psychologically driven feature... driven by some kind of awkward, socially obsessive mindset.
I use iMessage, WhatsApp and LINE are really big around me too. All three have this feature, and it's clear that FB would rather have us use their short messaging infrastructure instead.
So even if it's silly, the competition started being silly.
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.
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.
At the time, I believe the purpose was to engender a sense of trust in "the new medium of email." Obviously that's not the purpose of the implementation on FB.
So, what is the purpose?
Maybe it's the extremely late hour, but my cynical feeling is that this seems really a psychologically driven feature... driven by some kind of awkward, socially obsessive mindset.