The hacker Nik Cubrilovic has revealed that Facebook
collects data from users who have logged out.  Facebook confirmed but
insisted the information is only used for security purposes, aggregate
statistics or it it not logged at all, the Wall Street Journal reports.



Facebook data collection works by storing small files called "cookies”
on your computer when you visit their site.  Some cookies linger after
you log out.  The cookies relay information from any site you visit that
contains a link to Facebook such as a "like” button back to Facebook.
With this data, Facebook can map out your web usage.



http://dailycaller.com/2013/02/15/facebook-tracks-data-from-users-who-have-logged-out/