Amazon's Appstore is posting massive growth this year. A large reason for this large success is due to the popularity of the its Kindle Fire line of tablets, which do not have access to Google Play, so the Amazon Appstore is their only option. Amazon also has great developer features that are helping to spur the large growth.
Reports indicate that downloads in the Amazon Appstore are up 500% for this past year. Amazon made the news official when it announced that, "downloads in the Appstore have grown more than 500 percent over the previous year.” The Amazon Appstore first launched in March 2011, but it really took off in September of the same year when Amazon launched the original Kindle Fire.
The Kindle Fire line has grown to multiple devices since the original. However, it's not all tablets, because there are a few phones that come with the Amazon Appstore preinstalled, which could also be helping the growth of the Appstore. Amazon also offers free premium downloads from time to time in an effort to pull in users on other Android devices.
As I said previously, Kindle Fire devices do not support Google Play, which certainly helps stack the deck in Amazon's favor. Not that there's anything wrong with that; if a company has a popular product there is nothing wrong with using it to leverage something else in the line. With the popularity of the Kindle Fires continuing to grow, Amazon is likely see its Appstore continue to blow up as well.