Churn rate

Churn rate is cancellations in a period divided by the members you had at the start of it. A site with 500 members that loses 25 in a month has 5% monthly churn.

Churn compounds against you: at 5% monthly, roughly half your members are gone within a year, so growth has to outrun it before net member count moves. Reducing churn is usually cheaper than acquiring the equivalent number of new members.