Spotify took the wraps off a set of improved discovery features tonight at its ‘What’s Next’ press event, while revealing a spate of stats on its growth, and bringing Metallica’s Lars Ulrich and Napster co-founder Sean Parker together, in friendship, at last.

Stats first: Spotify now has more than 5m paying subscribers, with 1m of those in the US. CEO Daniel Ek claimed that Spotify is now the biggest subscription music service in the US – something that may ruffle feathers at rival Rhapsody, which announced its own 1m milestone in late 2011.

Ek also said that Spotify has now paid out $500m to music industry rightsholders. “Spotify pays back nearly 70% of all the money that we get in to rightsholders,” said Ek. “We’ve now paid out more than half a billion dollars, and that’s doubled in the last nine months.”

The new discovery features include a Follow tab to help users follow individual artists, celebrities, music experts and other users, and also a Discover tab designed to significantly improve Spotify’s recommendation features, suggesting new music based on users’ habits. Users will also be able to build their own “collections” on the service, adding albums to an online collection rather than having to save them to individual playlists.

The new features will initially roll out in Spotify’s desktop app, and its website client when it launches early next year. Other platforms – i.e. Spotify’s apps – will follow, although the company isn’t saying when.

Ek took a leaf out of partner Facebook’s playbook when describing the impact of the new follow and discover tabs. “Today we’re creating the music graph. Musical influencers come from all different places,” he said.

The biggest surprise was the joint appearance of Ulrich and Parker, given Metallica’s famous lawsuit against the original Napster in 2000. The pair have recently settled their differences, with Metallica also adding their entire back catalogue to Spotify today.

“We felt that we had much more in common than we realised at that time. Us sitting down together was long overdue,” said Ulrich, before harking back to the original battle. “The control option had been taken away from us, and then it turned into as far as we were concerned a street fight. You fuck with us? We will fuck with you! Let’s meet in the back alley over there.”

Daniel Glass, of Mumford & Sons’ US label Glassnote Entertainment, also appeared on-stage at the event, praising Spotify as an increasingly important income stream for artists, despite criticism from some musicians over their streaming payouts.

“I tell artists if you aggregate your catalogue, and we look at it over a period of time, it’s going to be very very significant income,” he said. “It’s showing up on our statements as something that gets bumped and bumped and bumped every month. You’ve got to play the marathon game.”

But Ek – who at one point described Spotify as “Punks. I don’t mean the kind of punks that are up to no good, but the punks that are up against the establishment” – said the service is keen to sign up other artists who have been holding back from Spotify.

“I want every band in the world to be on Spotify. I want all the music there. We just want people to listen to more music and share with their friends.”

This article was changed to reflect the fact that Spotify has 5m paying subscribers, not 5bn. Yes, we’re ashamed…