You need a metric that specifically answers this. It can be “x people did 3 searches in the past week”. Or “y people visited my site 9 times in the past month”. Or “z people made at least one purchase in the last 90 days.” But whatever it is, it should be a signal that they are using their product in the way you expected and that they use it enough so that you believe they will come back to use it more and more.
…manager might be responsible for communicating between designers, developers, and business leaders. But the skills required to be a designer, a developer, or a business leader are very different from the skills required to create alignment between designers, developers, and business leaders. UX, tech, and business are areas of subject matter knowledge that might be relevant to some product managers, but they are not the actual skills required to be a great product manager.