Music streaming has been a hard business in which to succeed. Companies like Pandora, Radio.FM and Grooveshark got the attention of consumers, however, one service rose above the others -- Spotify. The six-year-old music streaming service announced Monday that it had reached a major milestone.

In a blog post on Spotify.com, the team behind the streaming service thanked its fans and announced it had reached over 60 million users, with 15 million of them paying full subscriptions for the service. This expansion has occurred very quickly. Spotify announced last May that it had 10 million paying subscribers and 40 million users. If this exponential rate of user growth continues, it's likely that we'll see Spotify hit 80 million users and 20 million paid subscribers before the end of the summer.

Was this growth just the effect of time and consistently improving the service? That's highly unlikely. Quartz notes that its possible the subscriber growth was sped up because a deal Spotify offered in December for three months of Spotify Premium for 99 cents. Spotify's choice to let users stream music on their phones may have affected the subscription rate as well.  However, the fact that Spotify hasn't released growth numbers on a consistent basis makes it hard to determine any real patterns. 

On top of the general data points, Spotify still has to deal with some of its past problems. Cult of Android noted that "Despite its popularity, the service still has long had to contend with heavy losses due to hefty music royalty rates. Its U.K. business didn't record its first profit until October 2014 - six years after it made its initial debut."

There's also the issue of properly paying artists. In 2013, Spotify revealed that its going rate for music was $0.007 a song stream, which isn't enough to support most artists. This rate (among other things) is what caused Taylor Swift to pull her music from the streaming service.

It's uncertain if Spotify will ever be able to reconcile these problems. But if the current rate of subscriptions continues, then this streaming service likely has a long and hopefully successful future ahead of itself.