Facebook’s cookies track you across the web. Here’s advice on how to curb Facebook’s appetite for your information.
Most people don’t realize that Facebook can continue to monitor their internet activity, even if they are no longer logged into the site.Using "Facebook Connect," and other social plug-ins, Facebook is able to set up a cookie on any site that has a "Like" or "share" button, giving Facebook access to a startling amount of user information. Technically, the purpose of these plug-ins is to authenticate users, but it still has the ability to collect personal information such as the IP address of your computer, browsing data, outside login information, phone numbers, etc.
via How To Stop Facebook From Tracking You – Business Insider.