For visitors that are not Facebook users, the cookie contains a unique identifier, and it has an expiration date of two years. Facebook users receive additional cookies that identify them uniquely. Once those cookies have been set, Facebook will receive them for every subsequent visit to a website that uses Facebook’s social widget. That applies whether or not the Facebook user is logged in to his or her account and whether or not the visitor to the third-party site actually uses the social widget.
via Report: Facebook tracks all visitors, even if you’re not a user and opted out | Ars Technica.