Focusing on only one engine of growth at the time is much better for a startup. But of course, in order to grow your startup, you need the metrics in place, and to avoid vanity data. Focus on numbers, not on feelings.
The Sticky Engine of Growth.
The metric that you need is the churn rate - how many customers stop using your product every month.
The Viral Engine of Growth.
The metric you need is the viral coefficient - how many new users start using your product, because of the reference of a current customer.
The Paid Engine of Growth.
The metric you need is the lifetime value (LTV), and cost per acquisition (CPA). Make the LTV much higher than CPA.