This paper looks at the use of online data, especially social media metrics, to assess media audiences, with particular focus on musicians. It shows how audiences are defined by different sets of people, and grounds the use of social media to understand audiences in the history of mass media audience measurement. The second half of the paper focuses on visible social media metrics — likes, followers, and other such counts — outlining their appeal as measures and highlighting their fallibility and ambiguity. Throughout, the paper argues that different people construct information systems to collect, store, analyze, and interpret data, and that these are shaped by value systems. Metric and big data analysis generally serves economic values, while other approaches to data may be more appropriate for assessing social and personal values.