...But Web sites have this significant advantage over brick-and-mortar stores: They can track everything their customers do. Every page you visit, every purchase you make, every item you rate — it is all recorded. In the early ’90s, scientists working in the field of “machine learning” realized that this enormous trove of data could be used to analyze patterns in people’s taste. In 1994, Pattie Maes, an M.I.T. professor, created one of the first recommendation engines by setting up a Web site where people listed songs and bands they liked.