Barney, I too just got that the other day. It seems that whenever you click "like" on a topic or post, that is logged on your system as a "subscription" as well as a message in your inbox. I just deleted a whack of them, but am not sure how to clear the slate.
I eventually figured that out Sean, the message centre is not very user friendly. It displays the number of messages but everything else is blank until you click on it. I found over a thousand in "Sent", also after deleting everything you need to empty the garbage to permanently delete them which means clicking on everything again. There must be an easier way??