Google, Facebook and Twitter have made clear statements that this data is not used for tracking. ( see http://online.wsj.com/article/SB1000142405274870428150457

Google, Facebook and Twitter have made clear statements that this data is not used for tracking. ( see http://online.wsj.com/article/SB10001424052748704281504576329441432995616.html http://online.wsj.com/article/SB1000142405274870428150457632... and http://www.google.com/policies/privacy/ads/#toc-serve http://www.google.com/policies/privacy/ads/#toc-serve ). So if it does (and I am not saying it doesn't) they would be outright lying. Did anyone actually try to investigate if or to which degree this happening? It should be actually be possible to see if there is some correlation between the site you visit and the advertising you see.