WhatsApp just works and it works really well - you don't have to sign up, you don't have to add contacts. Isn't it obvious what a big deal that is? Also it is really reliable and fast. Often it seems even when my phone has only the slowest GPRS internet connection available to it, WhatsApp is still as good or better than texts in terms of speed and reliability. Facebook messenger probably wouldn't even connect in that situation in my experience. I'd be surprised if Hangouts wasn't the same
