When Spotify first debuted its Discover Weekly personalized playlist in July, it felt like a hit. On Twitter and in real-life conversations, even casual music listeners could routinely be heard singing its praises. Well, now there are some numbers to back up the hype: In its first five months, Discovery Weekly resulted in 1.7 billion streams.

In the quest to crack the code of online music discovery and curation, plenty of companies have thrown plenty of ideas up against the wall, always involving some blend of machine and human smarts. So what makes Discover Weekly different?

“We brought discovery to people in a very familiar format, which is the playlist,” says Matthew Ogle, Discover Weekly product owner at Spotify. “We took a lot of the work and cognitive load out of discovery. You don’t need to learn a new interface. It just works.”

And it’s not just the music geeks who are eagerly diving into their weekly music recommendations. When newer, more casual listeners try Discover Weekly, they listen about 80% as much as hardcore super-users.

For the unfamiliar, Discover Weekly is a playlist of songs that automatically appears each Monday in every Spotify user’s account. It analyzes that person’s listening history, focusing on the music he or she has played recently. It then compares that insight to the playlisting behavior of others. Scanning millions of playlists, the system finds tracks that are commonly listed alongside music with which a user is already familiar, and then groups those tracks together into a new, personalized playlist. It essentially takes the tried-and-true “people who like that, also like this” logic of collaborative filtering and applies it to the process of making a mixtape.

The end result, Ogle tells me, is a playlist of songs designed to feel like it was handcrafted for you by a good friend. Thanks to the magic and scalability of machine learning, Spotify is able to create these playlists for millions of users at once.

Much of the technology that powers Discover Weekly has lived within Spotify for quite some time. But until July, it mostly resided within the depths of the “Discover” tab of the service, where new albums and artists are laid out in a grid, sort of the digital equivalent of browsing the bins of a record store guided by a music-savvy best friend.