Google started helping users manage their photos back in 2002 with Picasa. Now, it's all about Google Photos, and that service has hit an important milestone. After four years of operation as a standalone service, Photos has a billion active users. Most of them are getting free storage from Google, too.
The Google Photos we know today started its life as part of Google+ back when the company was attempting to cram every service it could into its now-defunct social network. It abandoned Picasa in favor of Photos, and that turned out to be an excellent move. Google Photos broke free of Google+ a few years ago, quickly amassing a huge number of users. In 2017, the Play Store app reached the 1 billion download threshold, but that's different than having a billion active users.
Google Photos continues to offer free (compressed) photo storage to everyone, and most of Google's Pixel phones have free original quality photo backups. We expect that to continue with the upcoming Pixel 4, but Photos is popular with iPhone folks, too. It's just that good. Google has also launched Gallery Go, which focuses on offline image and video management. Google says that app is about the "next billion" users.