Nik Cubrilovic on the recent spat of Facebook privacy issues:
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.
He says he first noticed the problem in November of 2010. In case your didn’t know the standard practice is to delete the sites cookies when you logout of something — Facebook just modifies those cookies from what Cubrilovic found.