That said, Spotify has been running discounted deals for the better part of that time period, so basically no one is paying "retail" for their subscription. The big question is, what will happen when the prices go back to normal?
The other major looming question is, what will happen when Apple's Beats service is reloaded and launched this year, along with YouTube's Music Key? Will the increased competition stunt Spotify's growth?
If I had to wager on it, I'd say that all of those concerns will factor into a tougher year for the platform in 2015.
Here's a infographic from Statista that shows Spotify's recent growth.

You will find more statistics at Statista