WhatsApp co-founder Jan Koum recently tweeted that WhatsApp has crossed the 1 billion download mark on Android. The company achieved this with 4 engineers and Jan’s cofounder Brian Acton.

As of 2014 there were about 1.6 billion Android smartphones and 200 million Android tablets in circulation, for a total of 1.8 billion Android devices. This figure is likely north of 2 billion right now.

Even if we allow for the fact that some of WhatsApp’s downloads are subsequently removed, and that some WhatsApp apps that remain on mobile devices aren’t actively used, 1 billion downloads out of 2 billion devices is a huge number.

What’s the reason for WhatsApp’s success? I believe the answer is simplicity.

Beyond SMS, which I rarely use, I have three messenger tools on my smartphone. These are WhatsApp, Viber, and Skype. I use Viber to interact with my parents because this is what they prefer to use, and I use Skype for video conferences.

I believe that the reason why I prefer WhatsApp for messaging over the other platforms is because of its messaging centric approach and the amazing performance advantage that this laser focused approach gives WhatsApp over alternative services. Although Whatsapp also recently began offering calls, they do not lie at the core of the app. Messaging does. The fact that Viber positions calls as its core feature, and that Skype supports calls and video chats makes them much less reliable and slower than WhatsApp.

Even though the dropped calls which occur on Viber and Skype have as much to do with the performance of the telco networks that they’re taking place on as the performance of the underlying services, I hold Viber and Skype responsible for these dropped calls. Extrapolating the call performance of these services to their messaging performance causes me to treat their holistic performance as being unreliable. Although this results in a biased assessment, it’s a mental shortcut that I think many users take.

Because of the fact that they also accommodate more data intensive communication channels like calls and video chats, and position these communication channels as core features, Viber and Skype messages are delivered (and received) a split second later than WhatsApp messages. This split second makes all the difference for a user’s experience.

WhatsApp focuses almost exclusively on text messaging, the single most frequently used communication channel. By doing so it is able to achieve best in class performance in its delivery of this feature while also avoiding being negatively associated with the performance issues of the non-messaging features of multi-channel communication apps. This is simplicity at its best.