Facebook may be tracking your every move online even if you have never been on the site.

Not content with monitoring the movements of its own users, the largest social network in the world is building secret files on the activities of billions of people.

Mark Zuckerburg's company says that is uses this information to target adverts and content based on your preferences, as well as for security purposes.

Facebook account holders are able to download a copy of the file kept on them, which contains detailed records of their activities while logged in.

The privacy of users tracked via-third parties is currently less transparent, with no way of checking exactly what Facebook knows about you.