I know the websites you visited 15 years ago
People may be using it wrong. My browser cache clears every exit (likely for most people at HN). I had a new session going so it didn't detect where i went because there was no history. When i went on facebook and tried it again, facebook showed up. So the attack does work in some cases.
The one good thing that could come out of this is that websites with the gianto bar of "follow me/like me" icons could be different sized based on if you detect that the user has went to certain social media site.