Duolingo, which was also driven almost exclusively by word of mouth, optimized for free user growth: “By offering all of its learning content for free, Duolingo built a massive user base through word of mouth and no paid marketing. This also helped us build a growth revenue loop: Building a free and fun product → more users → more subscribers →... See more

- ABCMouse is making north of $100M/year with a $7.99/month product