keyplyr - 11:08 am on Nov 11, 2012 (gmt 0)
How many of these hits are simply trying to access all of your entire /images/ folder - no content specified?
In the 2 years I've been using FB as a traffic generator, I have never seen an authentic FB UA attempt access to an image directory without the specific image file used on the page... and I keep a very tight lookout on my daily logs, especially from major traffic sources.
I'm guessing this is one of the APs added by FB users (pinterest, simcity, etc) which would all get a FB IP and UA (facebookexternalhit/) since FB controls all activity from within its UI.
However, this may be something new I haven't seen yet. If it happened at my site, I'd complain to FB.