And that is why they succeeded imo. Because Facebook bought Whatsapp for it's 1 Billion users, and Whatsapp would never have hit that mark had it not been for the fact that they focused on market outside the US (where cheaper phones with android, candybar and other OSes were more popular).
While Whatsapp is still not as big in the US, it's literally the defacto mode of communication in countries like India, Brazil etc. to the point where "Msg me" would automatically mean "Msg me on Whatsapp"
That + the fact that it was tied to your phone number was in my opinion genius. I for a thousand years would not have though too much about Global Market (because I'd be focused on revenue and users in 1st world countries would be generally the ones with spending cash for messaging apps).