Magic numbers, such as Facebook's "seven friends in 10 days," serve as simplified, memorable mantras that guide companies in prioritizing efforts to improve user retention and product growth. These numbers are not precisely derived from data analytics but rather represent an understanding of when users begin to find value in a product, thus encouraging them to return. While many companies, including Twitter and Slack, have identified their own versions of magic numbers to boost user engagement, the concept remains more of an inspirational tool than a precise scientific formula. The effectiveness of a magic number lies in its ability to unify teams around a core mission, helping them focus on enhancing user experience and achieving retention goals. Ultimately, despite being an illusion, these magic numbers are valuable for creating a cohesive story that aligns with a company's growth strategy and user engagement objectives.