Airbnb cared about nights booked because each booking meant a guest found a place to stay and a host found a customer. Facebook’s growth team found that people who added seven friends in their first 10 days were more likely to stick around. And honestly, that makes sense. Your feed is probably more interesting when people you know are on it. Slack had its own number. When the company looked at its data, 93% of teams that had exchanged 2,000 messages were still using it. That suggested the teams had spent enough time on Slack to see if it worked for them.
The tricky part is what happens when a company gets too focused on the number. Airbnb could push guests to book longer trips, even if they only need a place for a weekend. Facebook could tell people to add a bunch of strangers, and suddenly their feed is full of people they don’t know. Slack could encourage more messages, which sounds pretty annoying when you’re trying to get work done. The numbers might go up, but users might have a worse experience. That’s basically Goodhart’s Law. Once a number becomes the goal, it can lose track of what made it useful in the first place.
