Duolingo has a unique approach to measuring success, focusing on "D1 retention," or the percentage of users who return the day after signing up. Starting with a 15% D1 retention rate at launch, Duolingo has since improved to over 50% today by prioritizing this metric.

One factor that contributes to user engagement is the use of gamification

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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