Why is any of this important—why do these Web giants want to catalog your interests and relationships? There's an obvious answer—because doing so allows them to sell ads targeted to you—and a less obvious one: Social signals are becoming the primary organizing structure of the Web.
Today most of the links you see online are determined by editors or algorithms—that is, by people who create sites manually, or by Google, which uses computers to guess what you might be interested in. But both these methods are imperfect. The people who create Slate's home page every day are just guessing what you'd like to read. Google, meanwhile, serves up thousands of links in response to your query, and even though it's often right, it's far from perfect. Web companies see social data as the solution to this problem: That trail of Likes you're leaving around the Web forms a picture of your deepest desires. With this picture, sites of all kinds—news sites, shopping sites, travel sites—can tailor themselves to your interests. In five years' time, you and your dad may visit the Huffington Post and not see any stories that overlap. The site will know, based on your social network, exactly what you want to read.