Facebook’s Chamath Palihapitiya famously found their metric in discovering that if they could get a user to “7 friends in 10 days”, they were much more likely to become an active lifetime user. Dropbox discovered that 1 file upload per folder led to higher engagement, and Zynga focused on “day 1 retention”. This type of analysis has traditionally required a sophisticated data engineering and machine learning infrastructure.

https://blog.clearbrain.com/posts/discover-your-products-7-friends-in-10-days