Looks like this is a response to the privacy breach reported last month by the Wall Street Journal: <a href="http://online.wsj.com/article/SB10001424052702304772804575558484075236968.html" rel="nofollow">http://online.wsj.com/article/SB1000142405270230477280457555...</a> (HN thread <a href="http://news.ycombinator.com/item?id=1801898" rel="nofollow">http://news.ycombinator.com/item?id=1801898</a>)<p>From that article:<p><i>It's not clear if developers of many of the apps transmitting Facebook ID numbers even knew that their apps were doing so. The apps were using a common Web standard, known as a "referer," which passes on the address of the last page viewed when a user clicks on a link. On Facebook and other social-networking sites, referers can expose a user's identity.</i><p>From this article:<p><i>Sadly, all those parameters [identifying the Facebook user etc] go in the URL [by which your Facebook app is invoked] and if you app includes any other external resources (iframes, imgs, scripts, etc.) that sensitive data gets passed along to them in the HTTP Referer header.</i>