The debate goes on as to how good subscription-based streaming services are for the music industry, but it's clear that adoption is on the rise. While Spotify revealed that it had reached two and a half million paying subscribers at the end of 2011, it's already hit the three million milestone before January is out. That's three million people making monthly payments of either $4.99 / €4.99 / £4.99 for ad-free listening, or $9.99 / €9.99 / £9.99 for mobile streaming — over 20 percent of the service's active user base. Of course, the most potentially significant driver of Spotify's growth is its recent launch in the US, along with its integration into Facebook, and managing director Ken Parks told the Financial Times that the plan is to keep expanding into new markets.