English learning

Duolingo has been the top education app in Apple’s App Store for most of the past decade, making the company a revenue of around $500 million annually. But most people who use the language lesson app do not pay. So how does a free educational app manage to have a valuation of $7.7 billion?
So how is Duolingo using AI-enhanced experiments to subsets of its user base, tailoring its offering to hook users like an addictive video game?