Music streaming giant Spotify has seen major traction from their “Discover Weekly” feature, which generates a personal playlist for each individual user based on their previous week’s listening habits. It’s basically a mix tape of old, new, and undiscovered tunes that cater to your personal style. No one receives the same playlist. Every Monday morning users wake up to a new catalog of two hours worth of music.

In fact, Discover Weekly is so popular that more than 40 million users have tried the feature, which comes out to more than five billion songs played. Since Spotify currently has 30 million paid subscribers, you can safely assume that each paid user has listened to the feature at least once.

Spotify believes its popularity lies in the new music aspect, as many of their subscribers listen to obscure music and become fans of unknown bands due to their exposure from the streaming service.

Their success also comes from a change in format that, until recently, was underused in music streaming. Most playlists (on all streaming sites) are updated manually by the users themselves, and many of the recommended playlists are aggregated to replay songs you’ve listened to a hundred times before, or “change” it up by adding in a different song from an artist you frequently listen to. Discover Weekly stands apart by always evolving and updating to fit each listener’s needs.

According to TechCrunch, Discover Weekly is most popular with the 25-34 age group, or what Spotify’s lead engineer Ed Newett calls the “after college” crowd. Newett says this age group is embracing the feature because it’s much harder to discover new music as our social circle changes.

Time will tell if other services, such as Apple and Tidal, will follow suit with similar features of their own.