You're a PM at Meta. How would you measure the success of Facebook's notification bell and its drop-down menu?

Focus on identifying key performance indicators (KPIs) that directly reflect user engagement and satisfaction with the notification bell and drop-down menu. Consider metrics such as click-through rates, user retention, and frequency of use. Discuss how you would segment the data to analyze different user demographics or behaviors, and think about qualitative measures like user feedback or surveys. Additionally, outline how you would set benchmarks for success and the potential impact of these features on overall user experience and platform engagement.