“I couldn't tell you what our daily active user number is or our monthly active user number because it's not what drives our growth model we are very focused on helping our users achieve their goal which is to learn a language and so at Babel we do have a proprietary metric that we optimize for it's called learner success.”

Optimize for Your Customer's Goal Metric, Not DAU/MAU — Babbel Uses "Learner Success"

Babbel deliberately doesn't track DAU or MAU — not because they can't, but because it's not the customer's goal. The customer's goal is to learn a language, so Babbel invented a 'learner success' metric that measures that directly. Stephen's principle: every app should define a metric aligned with what customers actually want to achieve. Optimizing for a metric that isn't in the customer's best interest puts the business on the wrong path, regardless of how large the numbers get.