Has Google had the ability to track users across multiple domains?

[dickbaker]

In a non-webmaster forum, I made the comment that Google has long been able to follow users from its search results to the site the user chose, and even beyond that site to the next site the user goes to, collecting information all the way.

Somebody called bull on this, but I remember reading about it here.

Am I nuts, or has this been going on for a few years now?

[Web_speed]

Well yes and no.

1) Yes if the user is using chrome or has the spybar installed on their browser or the "googleupdate.exe" service running on thier machine OR/AND the visited site landing page has one of Google services installed like ads/analytics/search/+1 buttons etc.

2) No if the user has none of the above and the landing page has none of the above Google services installed.

Judging by the poor state of the web nowadays my guess is that they are probably able to track more then 50%-60% of users online. Way too many infected websites, PCs and mobile devices out there unfortuntly...

[Pjman]

My really worry is that they are using adsense data to learn too much about my site.

I mean if they compare 5 sites (All have adsense). Wouldn't it be in their best interest to list the site with the highest adsense CTR as the first result?

In my niche I see a great deal of this.