Where does Safari store material from "fbcdn.net"?

If I **disconnect from internet**, launch Safari, open Preferences→Privacy→Manage website data, and click Remove All, everything vanishes **except** a cache from fbcdn.net. Selecting that only and clicking Remove also has no change. I know that belongs to Facebook, but why can't I get rid of it? Internet was disconnected, and no tabs were open, so it can't (shouldn't) be that Facebook instantly reloads it. Trying to find it failed. sudo find / -print | grep -i … with various things (fb, cdn, facebook, etc.) for … turns up nothing that seems like a possible fit. (Though it does show a heck of a lot of cruft from apps I haven't used in years!)