What’s the best way to measure product usage?

Donald T. Campbell (American social scientist)
There’s a Name for the Big Flaw in Our Obsession With Assessment and Metrics

The more a given metric—say, a national college ranking—is used to evaluate performance in some domain, the less reliable it becomes as a measure of overall success. Why? The people whose performance is being measured will neglect other parts of their job just to focus on boosting the relevant numbers, sometimes to the point of cheating.