They have tracking cookies installed on most user's machines. This is because they have such a high penetration rate of advertising inventory which they use to

They have tracking cookies installed on most user's machines. This is because they have such a high penetration rate of advertising inventory which they use to profile you by age /sex / interests. Then it's just a case of marking you as an HN user, they do this by detecting the referrer when you arrive at one of their sites.