Athletic clothing company Under Armour announced last week that it will be buying MapMyFitness, a freemium subscription app, for $150 million.While seasoned media professionals may look upon Under Armour as an usual buyer, the acquisition makes sense for both parties. For Under Armour, they’ve acquired “an open platform for digital fitness, a new direct sales channel, and a new way to stay present in its customers’ daily lives,” not to mention a way to stay competitive with Nike.MapMyFitness, meanwhile, is already suited to be paired with apparel sales. In June of 2011, Subscription Site Insider interviewed co-founder Robin Thurston about the company’s early success. At that time, the company had 3.6 million registered users and a 5% conversion rate to paying subscriber (that’s 180,000). 68% of revenues were from advertising, 30% from paid content, and 2% from eCommerce sales for a variety of sporting apparel and related gear.The Under Armour acquisition will allow MapMyFitness to increase this third revenue stream, since its 455% growth in free registered users (from 3.6 million in 2011 to 20 million today) is already pretty optimized. (Note: In 2011, MapMyFitness told us they had a 5% conversion rate from free user to paying subscriber. Assuming it maintained that conversion rate, we estimate it has 1 million paying subscribers today, and given its pricing plans, we estimate a minimum of $30 million in subscription revenues.)Those looking to copy MapMyFitness’s success should take a cue from the company’s platform agnostic approach, which allowed them to grow big without worrying about consumer buying trends, as well as present consumers with the benefit of multi-device integration. Plus, social media integration on every page allowed MapMyFitness to get 30,000 to 40,000 tell-a-friend referrals a day!You can read our in-depth Case Study on MapMyFitness on Subscription Site Insider.