JoachimSchipper is right that it doesn't have to involve cookies, and so are you that IP addresses aren't enough. Facebook could be using the same methods as P

JoachimSchipper is right that it doesn't have to involve cookies, and so are you that IP addresses aren't enough. Facebook could be using the same methods as Panopticlick (http://panopticlick.eff.org/ http://panopticlick.eff.org/), which are incredibly effective assuming the two users log in from the same browser.