Canadian rapper Drake is Spotify’s most-streamed artist of 2015, the streaming service announced Tuesday. But the true star for the company is a feature called Discover Weekly, which builds two-hour personalized playlists for each user every week. The playlists have racked up 1.7 billion streams since launching in July, though Spotify hasn’t disclosed how many of its users have tried Discover Weekly in total.

“It’s exceeded our expectations,” says Matt Ogle, Spotify’s product owner for Discover Weekly. When Ogle arrived at Spotify at the start of this year, the platform already had a “Discover” feature, but it was a rarely used hodgepodge of album and artist recommendations. He and his team decided to change their approach and focus on something more discrete and digestible. “You often might be in the mood for discovery but might not want to click through 20 albums,” says Ogle.

Playlists made the most sense as a delivery method, but also as a way to pick tracks that would align with users’ individual tastes. All the songs that appear on a Discover Weekly playlist are tracks that have appeared on some of the 2 billion other playlists available on Spotify. The idea, then, is to show you tracks that other people with similar musical taste are queueing up in their own playlists.

In this way, Discover Weekly is often able to surface songs you’ve never heard but are primed to take an instant liking to. In a given week, about half of Discover Weekly listeners save at least one of their playlist’s songs for further listening, Ogle says.

It took Spotify a while to find the sweet spot in terms of playlist length and variety. Early test versions of the Discover Weekly playlist were 100 tracks long, but listening to all those songs could feel like a chore. Another concept included only artists that a user had never streamed at all, but that made the playlists seem intimidating.

The company is still exploring new ways to improve Discover playlists. Currently, users’ recently played tracks matter less than their overall listening history for developing the playlists. But in the future, recent listens may play a bigger role for heavy users who are constantly trying new tracks and swapping between genres. The team also wants to give more weight to the way users actually interact with the playlists, so saving a song could give you more of that type of music, while skipping songs would eliminate similar tunes.

Discover Weekly’s success illustrates the next frontier for the big streaming services: providing entertaining, well-curated experiences, like a personal DJ. With massive song catalogues now standard across the industry, users are increasingly interested in tools that can help them surface songs they will actually enjoy, instead of being forced to wade through lots of noise. Through Discover Weekly, Spotify keeps customers tied to its platform rather than venturing over to explore rival services. “Our goal is to really make it feel like a kind of mixtape a friend might make for you,” Ogle says.

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