How Finnish disco killed my privacy

I noticed some unexpected activity on my Facebook wall the other day. I have a special list of "friends," who aren't really friends but more aquaintances. I have used that list to block them from seing much of what's going on on my Facebook wall (hey, we can still be "friends" right?). Now suddenly some of these people started "Liking" stuff I posted. And that struck me as..... weird.

What was going on? Turns out that I've been sharing my updates with all my "friends" since 31th of March. And I had no idea I was doing that — until I noticed that users I thought to be blocked had started interacting with my content.

After some investigation I found that ever since I shared a "Finnish disco" video on the 31th of March all my posts had been set to be shared with "Friends." That of course includes my blacklisted "friends." At first I couldn't understand how that was possible. I had read about the per post sharing settings when they first showed up, and here's Facebooks privacy policy dated 26th March 2010, explaining really well how it works. And I quote the relevant part:

Posts by Me. You can select a privacy setting for every post you make using the publisher on our site. Whether you are uploading a photo or posting a status update, you can control exactly who can see it at the time you create it. Whenever you share something look for the lock icon. Clicking on the lock will bring up a menu that lets you choose who will be able to see your post. If you decide not to select your setting at the time you post the content, your content will be shared consistent with your Posts by Me privacy setting.

Note that last sentence. If you don't select anything, it will go with your default setting! And my default setting was of course to share with "Friends," except for my blacklist. So what went wrong? Fast forward to today's privacy policy (called the Data User Policy, dated September 23, 2011). And I quote again: