You need a usage metric that trends over time. Kinda like an email list. You can see over time how many people subscribe, click or unsubscribe.
Depending on the product, you may want to look at transactions or something else.
If they are still paying you then you got to wonder what's going on. If it's a free service, then you need to figure out how the churn turns to $$ or whatever metric you care about.
You could use "Active (logged on) in last 30 days" or something.