Facebook has reportedly admitted tracking which sites its users visit even after they
log off, thanks to plug-ins and cookies.

Facebook, which has more than 800 million active users, also keeps close track of where millions
of non-members of the social networking site go on the web, even after they visit a webpage for any
reason only once, USA Today reported.

To do this, it relies on tracking cookie technologies similar to the controversial systems used by Google,
Adobe, Microsoft, Yahoo and others in the online advertising industry, Arturo Bejar, Facebook's engineering
director , was quoted as saying.

Here's how it works: Every time one logs onto Facebook it inserts a " session cookie" and a "browser cookie"
into one's browser. If one visits the site without signing up on the browser, cookie is inserted. From that
 point on, each time you visit a site which uses Facebook technology, the cookie alerts Facebook to
the date, time and URL of the page
you are viewing . The unique characteristics like one's IP address, screen resolution, operating system and browser
 version, are also recorded by the social networking site, it said.

Facebook spokesman Andrew Noyes said tracking systems are used to personalise content and help boost security.
 However, industry critics expressed concern.