Four of the biggest companies in the world — Alphabet, Amazon, Facebook and Apple — all reported earnings over the last two weeks. These companies either sell gadgets, services or ads — or maybe a combination of the three. And for a long time, it looked like selling ads, fueled by personal information, was the holy grail of digital business models. But lately, it looks like less of a slam dunk. Is the advertising star dimming as a reliable business model? Marketplace’s Molly Wood decided to ask an investor — Emily Melton, a partner at venture capital firm DFJ.