have you ever noticed that whenever you visit some sites while browsing social networking sites like “FACEBOOK” , “GOOGLE PLUS” “TWITTER” etc. its automatically detect that you are logged in and at point the you hit like button its automatically post that on your wall ,tweet or stream., may be you noticed .
then , know what happen behind the scenes
when you are logged into your Google or Facebook account and visit a movie review page on rottentomatoes.com, your browser automatically pulls in the “+1” button from Google’s servers and the “Like” button from Facebook. These downloads include the session cookies your browser uses to inform Google and Facebook that you have previously logged in, and so Google and Facebook automatically learn about your personal interest in the movie page you’re looking at, even if you never actually click on either of their buttons.
if you don’t want to get tracked then here is a solution
The Priv3 Firefox extension lets you remain logged in to the social networking sites you use and still browse the web, knowing that those third-party sites only learn where you go on the web when you want them to. All this happens transparently, without the need to maintain any filters. Priv3 is free to use for anyone.