Absolutely useless metric, both mathematically and economically. When a service has a lot of users, it has no room for growth and cost per user will be small.

Absolutely useless metric, both mathematically and economically. When a service has a lot of users, it has no room for growth and cost per user will be small. When a service has a few users, but has potential to grow exponentially, it will have high cost per user. Economically there's a large difference between services with respect to their monetization. A service where each user pays $1000 will have higher cost per user than a free service.