One of the promises of the Internet is the ability to target an exact individual. After all, unlike radio or TV, where messages are simultaneously broadcast to a broad audience, online advertising is served one impression at a time. Theoretically, if you know who the user is, you can perfectly target your message, or choose not to deliver it at all. A trend has recently emerged, whereby agencies can buy a "guaranteed" online audience, in the sense that the people exposed fit specific audience criteria, such as male teenagers, females 18-34, or auto intenders, etc.
