Retention

Retention measures whether users return and keep getting value over time. Acquisition can be bought and activation can be nudged, but retention is the metric that refuses to lie — people only come back to something that actually works for them.

I treat a flattening retention curve as the real test of product-market fit. Growth on top of leaky retention is just paying to refill a bucket with a hole in it, and that maths catches up with everyone eventually.