In the days since Spotify rolled out free mobile access to its on-demand music service, the company has seen an explosion in user interest. Earlier this week, Spotify saw its app download rate jump 400 percent since announcing its new free mobile streaming feature. The boom in user downloads has led to the app being ranked among the top 10 most downloaded apps in the iOS App Store in more than 20 markets worldwide, including the United States, United Kingdom, and most of Latin America.