What is the X-FB-DEBUG header returned from Facebook?
A little late, but just to add more information that I think it's useful. Facebook has now a place for it in its documentation, we can find it here
If you need to report a bug in the Graph API, we include a couple of additional request headers that you should send with your bug report to help us pinpoint and reproduce your issue. These request headers are X-FB-Debug and x-fb-rev.
To me, this seems to be a request tracking id, so that Facebook can find and trace an specific request details [faster] when you fill in a bug report. Have seen them in lots of bug reports on their bug tracker.