What's your products north star metric?

I recently attended a workshop and we were talking about the "north star metric". Even though this already seems to be yet another buzzword I found it a good question to think about on how you can actually measure the value your product delivers.

The North Star Metric measures the value you deliver to your customers. This means KPIs for your own like revenue or number of users are not a north star metric.

For example:

Airbnb: number of nights booked. Medium: total time spent reading


What would you say is the north star metric of your product(s)?