We operate a URL shortener, over the last week or so we've started seeing lots of weird requests for {normal url}/no_facebook_preview_picture.jpg from Facebook owned IPs and the user agent facebookexternalhit/1.0 (+http://www.facebook.com/externalhit_uatext.php)
If I post a normal link to our site on my wall (set as Only Me so I can test) I get the following entry in our access log
66.220.152.6 - - [05/Feb/2013:16:31:36 +0000] "GET /44_U HTTP/1.1" 200 1314 "-" "facebookexternalhit/1.1 (+http://www.facebook.com/externalhit_uatext.php)" "-"
However if I post a link that returns 404 or 410 (spam link removed after creation) I get this
69.171.237.15 - - [05/Feb/2013:16:49:16 +0000] "GET /notexistURL HTTP/1.1" 404 1319 "-" "facebookexternalhit/1.1 (+http://www.facebook.com/externalhit_uatext.php)" "-"
then within an hour or so
173.252.110.113 - - [05/Feb/2013:17:15:15 +0000] "GET /notexistURL/no_facebook_preview_picture.jpg HTTP/1.1" 404 0 "-" "facebookexternalhit/1.0 (+http://www.facebook.com/externalhit_uatext.php)" "-"
A WhoIs of that IP reports
NetName FACEBOOK-INC
NetHandle NET-173-252-64-0-1
So they are definitely Facebook IPs.
We're getting about 10-20 requests like this a day, all identical. We can only get 7 days worth of log files back but these requests were happening 7 days ago.
I've tested links that are unique, so there is no other way for anything to find that link. I don't personally use Facebook that much, and all except my test links were created/posted by other users but I recognize all the applications linked to my Facebook account and there is nothing unusual so I don't think this is a 3rd party app (I can provide a list if needed but they're all big name apps)
During my examining of the log files, Facebook doesn't even seem to be creating th