To optimize for growth, understand your funnel. In order for people to become regular users of your product, they have to pass through a bunch of hurdles. First, they have to be aware of your product. Second, they have to be interested enough to check it out. Third, they have to convert (download an app, fill out a form, confirm e-mail, etc.) Fourth, they have to do enough within your product to understand why it might be valuable in their lives. Fifth, they have to remember to come back. At each of these steps, you will lose people. If you can track and measure what that rate of loss is, you can then start to figure out where to focus your efforts to make your funnel less leaky.
To assess for product-market fit, look at retention. Do not look at the sheer number of people using your product or feature (which can be skewed by things like how aggressively you promote it.) Retention best correlates with whether your product is valuable because it tells you whether people who tried it liked it enough to return and use it again.