A small number of the world’s most valuable companies collect, control, parse, and sell billions of dollars’ worth of personal information voluntarily surrendered by their users. Google, Facebook, Amazon.com, and Microsoft—which bought LinkedIn for $26.2 billion in 2016—have in turn spawned dependent economies consisting of advertising and marketing companies, designers, consultants, and app developers. Some operate on the tech giants’ platforms; some customize special digital tools; some help people attract more friends and likes and followers. Some, including HiQ, feed off the torrents of information that social networks produce, using software bots to scrape data from profiles. The services of the smaller companies can augment the offerings of the bigger ones, but the power dynamic is deeply asymmetrical, reminiscent of pilot fish picking food from between the teeth of sharks.