Digital music streaming service Spotify, which launched in the United States roughly 18 months ago, has more than 15 million active users, including 4 million who pay for its premium services. Its secret? It makes listening to music social. When a consumer logs in to Spotify using his Facebook login, the service shares the tracks he listened to on the social network and on the Spotify web application. Consumers can see their friends’ most-listened tracks, recent playlists and what’s trending among all users. Consumers can create playlists and share them with their friends via Facebook, Twitter or e-mail with the click of a button. Or, a group of friends can create a playlist together, with each member able to add or subtract tracks to craft a unique soundtrack.

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