Did you know that Facebook gets a report every time you visit a site with a Facebook “Like” button, even if you never click the button, are not a Facebook user, or are not logged in?
That, and more you should be aware of here in this Consumer Reports article.
Priv3 for Firefox will silently prevent the cookies from Facebook, Twitter, LinkedIn, and Google+ from being installed in your browser, thus protecting you from being tracked everywhere you go online.
Take it a step further and install CSFire to block not only the big four social networks, but any site that surreptitiously makes requests to third parties without your knowing it. It takes a little more management to unblock sites you want to function this way, but it will greatly increase your overall online privacy (and safety).