Lisen. OneSignal is not only a Web Browser Push Notification website. It is for more than 50+ Different Services including Android, iOS Apps, Websites, Blogs. They support Xcode, Unity, Marmalade, Adobe Air, Phonegap, Android Studio, Visual Studio, Cordova, Cocos2d-x, Corona, Xamarin, Ionic, React Native Intel XDK Titanium and much more.
Moreover, their services are being used by big companies like Adobe, Uber, Mic, Line, Tom's Hardware, etc. and their Alexa Ranking is 1500 whereas others are having it somewhere around 5000 - 50000