Calling all three numbers North Stars hides their most interesting difference. Airbnb’s nights booked is a company-scale transaction metric. Facebook’s seven friends in ten days is an individual activation target. Slack’s 2,000 messages is a team-level adoption threshold. They sit at different points in the value chain.
Airbnb uses nights because they provide a common unit for booking activity and platform scale. The behavior is a guest-host match, measured net of cancellations and alterations. A growth team could push longer stays with coupons or tolerate poor listings that generate bookings. Nights would rise, but so could bad stays, support costs, and lost trust.
https://www.sec.gov/Archives/edgar/data/1559720/000119312520294801/d81668ds1.htm
Facebook used seven friends to represent the network density that made communication and the feed useful. The team found the pattern by looking backward from engaged users. The pattern does not prove causation. Aggressive imports or weak-tie suggestions could inflate friendships while making the product feel random and intrusive.
https://genius.com/Chamath-palihapitiya-how-we-put-facebook-on-the-path-to-1-billion-users-annotated
Slack used 2,000 cumulative messages to identify teams that had genuinely tried the product, not just created an account. It proxied repeated collaboration and a useful archive. Bots, forced check-ins, and message splitting could fake that adoption. The result would be notification overload, buried decisions, and muted channels.
https://review.firstround.com/from-0-to-1b-slacks-founder-shares-their-epic-launch-strategy/
Goodhart’s Law appears when a metric becomes easier to improve than the value it is supposed to represent. The trickiest part is that gaming can emerge from normal growth work. Coupons, onboarding prompts, and notification design can all change user behavior without anyone intending to manipulate the metric. I therefore think a metric remains useful only as long as it stays connected to the underlying behavior or value that made it meaningful in the first place.
