If I send a Facebook message when someone is offline and do not receive a "seen" notification for more than a week, what might have happened? Does it mean that the message was deleted without being read or does the "seen" notification go of after a week or what? is it possible not to get a seen notification even though it might be read? I know my friend has been online since then. Any ideas?
The "seen" blurb is not a superbly reliable indicator of whether or not somebody has seen your message. For example, to view a message while not letting the other person know that I did, I can just use Chrome Developer to preview the message. The other person could have also seen the message on their phone, perhaps via email, etc. They could have also read the message in a chat window that was still flickering.
The other person could have also archived the message, moving the message out of his/her inbox, while never having actually viewed it.