Building a product-driven business requires a focus on both acquisition and retention, with the latter being crucial for sustaining long-term growth. Retention is often considered the best measure of product-market fit, as it reflects whether users find ongoing value in a product. High retention rates reduce the pressure on acquisition efforts and are more cost-effective than constantly acquiring new customers. Retained customers provide valuable proof points and referrals that can bolster marketing and sales efforts. Product Managers should prioritize understanding and improving retention rates, as they play a significant role in determining a product's success and the company's overall growth trajectory. Effective retention strategies involve measuring user engagement at different stages and developing plans to enhance user retention over time.