indyank - 3:36 am on Oct 9, 2012 (gmt 0)
They could be using click-stream data that some ISP or other sells them, or maybe that they gather from free wi-fi that they provide. I don't know because I've never seen any privacy statements related to those services. They could be using DoubleClick cookies somehow, I guess.
That might be true.
I think I referred a link to a research paper here several months ago which tells us about how they could compute user metrics like bounce rate directly from the content! If what was stated in that research paper was true, they could very well be arriving at all other relevant metrics directly from the content! Then they could be using all the indirect influencers(signals) to validate the metrics they arrived at.