An information designer has information about consumers' preferences over products sold by oligopolists and chooses what information to reveal to firms who, then, compete on price by making personalized offers. We study the market outcomes the designer can achieve. The information designer is a metaphor for an internet platform which uses data on consumers to target advertisements that include discounts and promotions. Our analysis demonstrates the power that users' data can endow internet platforms with, and speaks directly to current regulatory debates.