Okay, maybe I was stupid but I couldn't find the link to the demo in the article. Author posted it somewhere here in the comments: http://homakov.github.com/fbdetect.html

So all this does is to check if my username matches with some preexisting username. Its no way you can detect my username if you don't already know it. Also even after I gave the demo my username, chrome simply blocked a popup and the whole thing failed...

Either I don't get it or this is a lot less impressive than the title suggests.