https://sarahtavel.medium.com/the-hierarchy-of-engagement-expanded-648329d60804#.j7apmssjf

• For growth, the most important element is users completing the core action (FB —> adding friends). Don't look at anything else.
• Completing core action is mostly correlated with retention
• Retention - Users will accrue benefits and mount loses as they use the product. Accrue benefits mean, they will discover and enjoy the product by realising its value and find new ways to use the product and if they want to stop using the product, they would lose more.
• The way to measure is cohort analysis
  • Growth - size of the cohort
  • Engagement - ratio of users performing the core action
  • Retention - cohort performance over time