A retailer (Amazon), social media company (Facebook), and information services company (Google) now capture more than half of all advertising spending in the U.S. The Inside Orchard essay “The Advertising Triopoly” focused on how the three were able to reach such a milestone: establish formidable ecosystems powered by not only software, but also customer data.

Today’s advertising empires are built on tracking our behavior and activity online. The degree of…