Hacker News was penalized by recent Google Panda update (11th April).
alexa.com
alexa.com
I submitted because Alexa is fairly accurate with very high traffic sites, and the drop in HN's rank and reach seemed to coincide with Google Panda roll out.
Edit: I think I found it, in mid-April there's a spike downward. But it recovers quickly, and it's not the biggest - change the view to 6 months or max and you can see some crazy changes at times.
I use Google to find tidbits of information. One-off questions. I can't think of a single site that I have ever discovered via google that is something I visit regularly.