I use this to block Facebook. So far, to the best of my knowledge, it has always worked perfectly.
Just now, I got linked to a Guardian article, and as it was loading, I noticed "connected to www. facebook .com" flash by on the status thing in the lower left.1 What? Sure enough, it got through (and presumably has gotten through elsewhere) far enough that Privacy Badger is aware of it and blocking its cookies (but not FB in its entirety). This seems to hold true for The Guardian's entire site. I checked /etc/hosts, and it is alive and well (including the www. facebook .com entry). I pinged www. facebook .com, and as expected, the request didn't leave this computer.
I turned Wireshark on the problem, capturing for about 10 seconds on loading a Guardian page (enough time to confirm Privacy Badger was aware of Facebook's existence on that page), but didn't find anything interesting. It clearly didn't capture a couple of other things PB noted and let through, though. All I can think of is something to do with CDNs, which were prominently represented in the traffic, but I don't know enough about. IP list is here. There weren't any noteworthy DNS requests in that capture, either.
I've attempted to mess with Firefox's debug tools, but there's a ton of garbage on this page cluttering everything up and I'm not always sure what I'm looking at. This (and a duplicate of it) is the only FB thing in net traffic as far as those debug tools can tell - it looks like an obvious pixel tracker that got blocked (by PB?), but I'm still not 100% sure what happened to it. The part that generated that request is obvious enough at least. It's just in HTML, directly below body. There's a stupid amount of source code for this page and I'm not sure where else to look for anything FB related.
I would say PB is blocking it before the DNS request is made based on the timings in FF's debug tools, but FB tries to track basically everything everywhere and with this hosts file I don't usually see PB being aware of it. Also, PB says it's only blocking cookies, so does that mean pixels too?
Any ideas what's happening here? I'll learn how to debug whatever needs to be debugged, I'm just not sure where to go next.
1edited for annoying auto link completion - ignore the spaces in the URLs up there