One of the first things that Google announced at its big event today is that Android just keeps on growing. Google CEO Sundar Pichai said on stage that at the moment, there are more than 1.4 billion active Android devices in the world. "Active" in this context means that they've been used within the last 30 days.
This number is up 40%, from 1 billion, back in June of 2014. Keep in mind that this doesn't represent the total number of Android devices that were ever sold, it's just those that are still active, or have been so in the last month.
The Google Play Store is now being used on more than 1 billion devices. Again, this means 1 billion phones and tablets have used Google's content store in the last 30 days. So it appears that the Play Store either isn't installed or simply isn't used on a staggering 400 million devices out there.