Back in May of 2016, Google announced two new messaging apps: Allo and Duo. The former, as you probably know, is being killed off in about three months. Duo, however, is still alive and kicking, as it's a pretty good video calling app. It just hit a whopping one billion downloads on the Play Store, which is no small feat.
Duo hit 500 million downloads just six months ago, so another 500 million in that time span is pretty impressive. Granted, Duo does come pre-installed on many Android devices, but it's basically impossible for any app that isn't pre-installed on some devices to reach that number. In fact, looking at Wikipedia's list of 25 apps with over a billion downloads (as of April 2018), the only app on the list that doesn't come pre-installed on any device is... Subway Surfers. Yeah, I didn't expect that either.
In case you somehow haven't ever tried out Duo and want to do so now, you can download it via the Play Store widget below.