Five billion is a big number, especially when it comes to app installs. To date, only pre-installed Google apps (Play Services, YouTube, Maps, etc.) have passed that milestone, but now Facebook has just joined the elite club.
The Play Store listing for Facebook now reports that the app has over five billion installations, coming almost exactly five years after it passed one billion installs. Facebook was the first non-Google app to pass that milestone as well.
It's not too surprising that Facebook is the first non-Google app to pass this milestone. Beyond being one of the most popular apps on smartphones period, Facebook has also made deals for years to pre-install the app on phones from Samsung, LG, Huawei (well, not anymore), and others.