Each of these companies picked one number to steer by. Each number was a stand-in for a behavior the company actually cared about, which is exactly what makes it easy to game. As Goodhart’s Law warns, once a measure becomes a target, it stops being a good measure.
Airbnb: nights booked
Nights booked works because every booked night means a guest found a place and a host got paid, so it measures value on both sides of the marketplace. A team could game it by pushing deep discounts or nudging long, cheap stays that guests later regret. Bookings would rise while trust, host income, and repeat visits fall.
Facebook: 7 friends in 10 days
Chamath Palihapitiya’s growth team compared cohorts and found that engaged users had reached at least seven friends within ten days of signing up. Friend count was a proxy for a feed full of people you care about. Spamming random friend suggestions would hit the number, but it would fill feeds with strangers and make the product feel empty anyway.
Slack: 2,000 messages
Stewart Butterfield said any team that exchanged 2,000 messages had “really tried” Slack, and 93% of those teams kept using it. The number stood for a team actually moving its work into Slack. Flooding channels with bot messages would cross the line without that habit forming, so the team would churn and add noise along the way.
The takeaway: a good metric only works while you keep chasing the behavior behind it, not the number itself.
