Have you noticed that some social media sites like Facebook have started showing you “friend endorsed” ads? They show an ad for this and that company with a little message that your friend likes this particular company. Or how about when you go to a website and in a sidebar they show you a bunch of ads for websites you have visited recently.
Many of the social media and search engine companies offer you some control over how your information is used and Mashable just release a quick and handy article on how to opt out of some of this data sharing and friend endorsements. http://mashable.com/2013/11/12/opt-out-data-tracking/