Measuring engagement on Facebook pages is challenging but important for marketers to prove the value of Facebook. While likes are a common metric, they do not indicate real engagement with the brand. More meaningful metrics include comments from "core fans" who engage more than average. Even celebrities struggle with engagement, so brands cannot expect easy success. Apps provide an opportunity to access richer user data that could inform marketing. However, proving return on investment from social media remains the greatest challenge due to lack of data linking activities to sales.