On tracking users online

Let's look at three companies from our list of As. Adnetik is a standard targeting company that uses real-time bidding. They can offer targeted ads based on how users act (behavioral), who they are (demographic), where they live (geographic), and who they seem like online (lookalike), as well as something they call "social proximity." They also give advertisers the ability to choose the types of sites on which their ads will run based on "parameters like publisher brand equity, contextual relevance to the advertiser, brand safety, level of ad clutter and content quality."

It's worth noting how different this practice is from traditional advertising. The social contract between advertisers and publications used to be that publications gathered particular types of people into something called an audience, then advertisers purchased ads in that publication to reach that audience. There was an art to it, and some publications had cachet while others didn't. Online advertising upends all that: Now you can buy the audience without the publication. You want an Atlantic reader? Great! Some ad network can sell you someone who has been to The Atlantic but is now reading about hand lotion at KnowYourHandLotions.com. And they'll sell you that set of eyeballs for a fifth of the price. You can bid in real-time on a set of those eyeballs across millions of sites without ever talking to an advertising salesperson. (Of course, such a tradeoff has costs, which we'll see soon.)

This article on how websites track user behaviour on the Internet.

Research psychologists have known for a while that the media you consume shapes your identity. So when the media you consume is also shaped by your identity, you can slip into a weird feedback loop. A lot of people see a simple version of this on Facebook: You idly click on an old classmate, Facebook reads that as a friendship, and pretty soon you’re seeing every one of John or Sue’s posts.

Gone awry, personalization can create compulsive media – media targeted to appeal to your personal psychological weak spots. You can find yourself eating the equivalent of information junk food instead of having a more balanced information diet.

Read this Author Q&A with Eli Pariser