The Wall Street Journal’s What They Know series documents new, cutting-edge uses of Internet-tracking technology. The Journal analyzed the tracking files installed on people’s computers by the 50 most popular U.S. websites, plus WSJ.com. The Journal also built an “exposure index” — to determine the degree to which each site exposes visitors to monitoring — by studying the tracking technologies they install and the privacy policies that guide their use. http://blogs.wsj.com/wtk/