Furthermore, based on a quick estimate based on my current Firefox cookies, Google is (through Google Analytics) aware of 80%-90% of my browsing activity.
83 karma · joined July 26, 2010
In theory, I see your point about Google not having direct access to a site's first party cookies, but Google is clever. If you monitor the requested resources when you visit any site with those cookies, you'll notice a request to http://www.google-analytics.com/__utm.gif? followed by parameters sending the values of those cookies to Google. So, they are tracking said cookies.
Analogously, if I measured academic skills and availability by time spent in libraries and time spent teaching, I'm sure we'd see a similar peak in 20's because grad students spend so much time doing both these things and productive professors need this less.