Funnel analysis
Define the steps (visit → signup → activation → first action → recurring use). Count users at each step. Look at conversion rates between steps. The biggest drop is your bottleneck. Fix the bottleneck before optimizing anywhere else — gains elsewhere are washed out by the leak.
Cohort analysis
Group users by when they joined (the "cohort"). Track each cohort's retention over weeks/months. Cohorts that retain better than older ones = your improvements are working. Cohorts that retain worse = something regressed. Aggregate metrics hide both.
↳ in the wild
Aha moment / activation
The action that predicts long-term retention. Facebook's famous one: 7 friends in 10 days. Slack's: 2,000 messages in a workspace. Find yours by looking at retained vs. churned users — what did the retained ones do in week 1 that the churned didn't? That's your activation event.