I would evaluate the effectiveness of Instagram's discovery features using a variety of metrics for a comprehensive understanding of their success. For example, usage metrics like daily and monthly active users can indicate the feature's popularity, while the number of impressions generated by the discovery feature can reveal its reach. Engagement metrics, such as the number of posts clicked on, likes, comments, and other interactions with the discovered posts, will provide insight into user engagement. Additionally, tracking the retention rate of users and the increase in followers for the accounts discovered are also valuable indicators. To gain a more complete perspective, conducting user surveys or collecting in-app feedback can help determine whether the feature is fulfilling user needs and expectations, thereby enhancing user satisfaction. Finally, monitoring a reduction in churn rate could suggest that users find value in the discovery feature.