The retention number that means desire is unprompted return.

Reforge argues that how often people should come back is a property of the problem, something you find by watching behaviour rather than something you set. Duolingo's biggest growth win was a streak-saver notification: frequency made by the product. Daily active users counts both kinds of visit the same.

The unprompted return is a truer measure of desire

References: Reforge, "Defining a Retention Metric" (members only). Jorge Mazal, "How Duolingo reignited user growth", Lenny's Newsletter.