diberry - 2:36 am on Dec 15, 2012 (gmt 0)
Jeff, that's extremely interesting data. It does stretch credulity as a mere coincidence.
Another possibility occurred to me today:
Despite quarterly earnings, Adwords (and indeed all net advertising) is under pressure to prove itself to advertisers. What if Google is feeding the best traffic to the sites that pay for Adwords, and the zombies are simply all that's left for you guys during certain periods?
And maybe this could explain the weird on/off effect - the sense you guys get that somebody flipped a switch and here came the zombies, that could just be when some major advertiser has started a major campaign.