We study an index to measure the popularity of artists in music streaming platforms. This index, which can be used to allocate the amount raised via paid subscriptions among participating artists, is based on the Shapley value, a centerpiece in cooperative game theory. We characterize this Shapley index combining several axioms formalizing principles with normative appeal. This permits to place the index in the literature, as an alternative to the well-known (and widely used in the industry) pro-rata and user-centric indices.