This topic sounds simple until you try to implement it for real users. Then the edge cases eat you alive.
Here's a scenario that kills startups:
Founder: "We're doing great! We have 10,000 users!"
Advisor: "How many are paying?"
Founder: "Well... 12."
Advisor: "What's your churn rate?"
Founder: "Uh... I'm not sure."
Advisor: "What's your revenue?"
Founder: "$348/month."
Advisor: "You don't have a business. You have a free tool with 10,000 tire-kickers."
10,000 users sounds impressive. It is meaningless without context. This is the vanity metric trap.