For the platforms I've built I've had access to the customers table for example, you know if it's growing or shrinking.
You don't even have to explicitly nose around, looking at the logs you'll see customer IDs and if they are sequential a keen eye will notice if they stop getting any higher.
We generated a report each week to let all the devs know how many customers we had starting that week, what products they were launching, how many users they each had...
It was great for planning the on-call rotation as well as giving us a heads up on which nights we could go out drinking and how late we could come in the next morning.
You don't even have to explicitly nose around, looking at the logs you'll see customer IDs and if they are sequential a keen eye will notice if they stop getting any higher.