Cookies can tell you, as well as any advertisers who want to market to windsurfing enthusiasts. Add to this the voluntary information audiences online might be willing to share with a given website. Your name, your age, sex, income, geographic location—in ways complex and yet not entirely appreciated, our online activities have delivered the holy grail of advertising from time immemorial: knowing exactly an audience’s interests so that the ad man can market only to the most promising leads.
