Facebook 'Like' button draws privacy scrutiny

"Widespread use of the Like button allows Facebook to track people as they switch from CNN.com to Yelp.com to ESPN.com, all of which are sites that have said they will implement the feature.
Even if someone is not a Facebook user or is not logged in, Facebook's social plug-ins collect the address of the Web page being visited and the Internet address of the visitor as soon as the page is loaded--clicking on the Like button is not required. If enough sites participate, that permits Facebook to assemble a vast amount of data about Internet users' browsing habits."

Read further...Facebook 'Like' button draws privacy scrutiny