This afternoon I posted on Facebook about a friend's Mustang and, not long after, noticed a Mustang ad in the right-hand column. This Wall Street Journal investigation explains how they do that. I heard the reporter, Julia Angwin, talking about it on NRP's Fresh Air not long ago.
The sites we visit most often, and advertising firms that specialize in this stuff, have ways of gathering information about the things we look at on line and generating ads that match our interests. Angwin found that the average site puts over 60 tracking devices on your computer when you visit. Often third parties are running the tracking programs without the host sites' even knowing.
Angwin said that when she checked out some shoes on line, "they kept following me around the web" as ads popped up at other places she visited. While she says that our profiles can get pretty detailed and sophisticated, our names aren't attached to them. The data is mined and traded instantaneously, since the whole idea is to get to us while we're in the market and before the data are corrupted by our fickleness, or the other people who are using our computers and start searching for guitar strings instead of pumps.
I tried to summon some outrage about an invasion of privacy inherent in this example of the market in hyperactivity, but I couldn't. This is Big Broker, not Big Brother. I only got worried when the NPR interviewer, Dave Davies, asked Angwin, "Is anyone in the government interested in this?"