It's not often that applications on the Play Store, or any mobile app store for that matter, hit over one billion installs. To be sure, this is quite a milestone that only the most popular of apps will ever be able to achieve, and there are only a small number of applications which have been able to pull it off thus far. A collection of the spots are perhaps unsurprisingly shared between Google and Facebook, and the latest app to pass one billion installs is Google's Play Music app. This might seem somewhat unimpressive, but you have to remember that not all of Google's apps come preinstalled on all Android devices these days.
Having said this, Play Music is one of the apps that does come pre-installed on many Android devices from different OEM's so the majority of those installs likely comes from those who purchase Android phone and activate the device. There are also a small number of devices running Android which don't have the app pre-installed and there have more than likely been some users who have chosen to download it. Yes, the billion installs mark is impressive, but there is no way of knowing how many people actually installed the app as opposed to simply buying a phone where it came pre-loaded.
Furthermore, there's also no telling how many people are using Play Music whether on a daily basis or casually throughout a period of time. In either case, Play Music has just joined a very exclusive club. If there's one thing this does display, it puts into perspective how popular the Android platform really is (if you were unsure in the first place). Aside from that, it shows that Google has Play Music pre-installed on a fairly large number of phones, which some users probably don't care for. Play Music being the fifteenth app to hit one billion installs joins other Google apps like Hangouts and Chrome, as well as apps like Facebook, Facebook's Messenger, and Whatsapp. If you haven't used Play Music yet or in quite a while, the latest few updates have brought the app forward to round it out into a pretty great little music app.