The company spun out Photos from Google+ at I/O 2015. It's the ninth Google product to hit the billion-user mark, following Search, Gmail, Chrome, Drive, Maps, YouTube, Play Store and Android, but it got there a lot faster than many of those. Gmail, for instance, took 12 years to add a tenth digit to its user numbers. Facebook and Instagram, meanwhile, each hit a billion users in about eight years.
One of the reasons Google hit the figure relatively quickly is because it frequently adds new useful features to Photos. The latest is Gallery Go, which is a lightweight photo manager for Android. It lets people manage their images offline and can automatically sort them by categories including people, documents and places.