...Arctan? Haha, what the hell is that?<p>This does well to illustrate what a fiendishly difficult thing it is for a typical Facebook user to maintain privacy settings, and how difficult it must be for Facebook to actually adhere to this rigamarole in every view, API, feed, et al. presented by their site. (And they do screw that up from time to time.)<p>Part of the reason I think they want to "simplify" privacy by making more stuff open is because it is such a royal pain to implement. Compared to Twitter, the logic that must go into every database query to determine viewability of friends or comments or pictures must be ridiculously complex. The current ability to group friends into lists is roughly equivalent to implementing ACLs on groups, which is just as much fun for the user to set up and maintain as it is for a programming team to get working (not fun at all).