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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-Great use of data and gamification to keep people engaged

Say you have 100,000 DAUs and a retention rate of 80%, and for every five users, one of them tells a friend each day to download your app (and they do it). You get 20k new users per day just from word of mouth. In this equation, Xn is the number of DAUs today and Xn+1 is the number of DAUs tomorrow:

X n+1 = (X n * 80%) + (X n * (1⁄5))

If you start... See more