It's not just the raw numbers. I would love to move onto Telegram, which is more secure, has an API, and works from a desktop. But, I can't.
There are about five or six odd people in my life (professional or private) that I message frequently. Even if 1 out of them isn't on a particular messaging platform, I can't adopt that platform wholeheartedly. With Fb Messages, Hangouts and every other popular smartphone oriented messaging platform there would always be that one or two contacts that you regularly communicate with, who would be left out due to having an incompatible device. Whatsapp on the other hand works on pretty much every phone. When you want to replace something as prevalent as SMS, the long tail matters.
There's another tangible benefit of focusing on so many platforms. Whatsapp, which doesn't spend much (if anything at all) on promotion, is prominently featured in the advertisements of many budget phones, as manufacturers see Whatsapp support as a strong selling point.
There are about five or six odd people in my life (professional or private) that I message frequently. Even if 1 out of them isn't on a particular messaging platform, I can't adopt that platform wholeheartedly. With Fb Messages, Hangouts and every other popular smartphone oriented messaging platform there would always be that one or two contacts that you regularly communicate with, who would be left out due to having an incompatible device. Whatsapp on the other hand works on pretty much every phone. When you want to replace something as prevalent as SMS, the long tail matters.
There's another tangible benefit of focusing on so many platforms. Whatsapp, which doesn't spend much (if anything at all) on promotion, is prominently featured in the advertisements of many budget phones, as manufacturers see Whatsapp support as a strong selling point.