agree with Retention, but both Activation & Retention together are important. you need to have an initial call-to-action (CTA) to USE and EXPERIENCE the product for the first time (or at least explain it / see how it works) in order to have a shot at Retention to then work.

IMHO the Activation-Retention loop is super important to get working -- at least at a basic MVP level -- before you start really driving hard on Acquisition, Referral, or Revenue.