Facebook never fails to surprise me. Just when you think you've got a handle on the damn thing, something else comes along and surprises you. Did you know that you've got another inbox, and that it's quite possibly full of messages from people which you haven't replied to?
If you go to your Messages option on the left hand side of the page and open it you'll see something that looks like this:
Click the 'More' option and you'll see you can look at Unread, Filtered or Archived:
'Filtered' is the option that you want. Click on that and you will see messages from people you don't know, have never had contact with directly and so on. I found about half a dozen from people this year. I've spent the last few minutes going back and forth with messages, all starting with 'I'm sorry that I haven't replied to you before but...' and then blame Facebook. (If in doubt, blame Facebook. If really in doubt, blame Facebook and Google. If totally screwed, blame the Internet.) Try it - let me know if you've found messages that you didn't know you had! While you're at it, check out the Message Requests option as well. You might find even more memos from people there as well. I don't know why Facebook puts some in the Message Requests section and others in the Filtered section, but I doubt that they are going to be telling us any time soon.