There was a time when any site that deposited tracking cookies on your system, we called it spyware. So how is Facebook getting away with tracking people who have logged out of facebook?

Yet another reason why Facebook will never get my real name and bio....

http://nikcub-cache.appspot.com/logg...-is-not-enough

The advice is to log out of Facebook. But logging out of Facebook only de-authorizes your browser from the web application, a number of cookies (including your account number) are still sent along to all requests to facebook.com. Even if you are logged out, Facebook still knows and can track every page you visit. The only solution is to delete every Facebook cookie in your browser, or to use a separate browser for Facebook interactions.
The primary cookies that identify me as a user are still there (act is my account number), even though I am looking at a logged out page. Logged out requests still send nine different cookies, including the most important cookies that identify you as a user

This is not what 'logout' is supposed to mean - Facebook are only altering the state of the cookies instead of removing all of them when a user logs out.

With my browser logged out of Facebook, whenever I visit any page with a Facebook like button, or share button, or any other widget, the information, including my account ID, is still being sent to Facebook. The only solution to Facebook not knowing who you are is to delete all Facebook cookies.