Only a handful of Android applications have reached five billion downloads, with most of them being Google's pre-installed apps. Facebook reached the landmark last year, likely thanks to its partnerships with Samsung and other OEMs, and now Facebook-owned WhatsApp has joined the elite club.
As with most Android applications that reach large amounts of installs, this number doesn't just include downloads from the Play Store, but also pre-installed copies. Samsung has bundled the app with some smartphones in the past, most notably the Galaxy S6, and Huawei has done the same (until recently).
Still, with WhatsApp being the primary means of communication in many countries around the world, and with over 1.5 billion monthly users, a large percentage of installs are probably regular Play Store downloads.