Similar situation here in Germany. Everyone and their mother uses Whatsapp. Most tech people underestimate the importance of not having to create an account or manage friends/contacts. Your phone number and your existing contacts is all it takes to set you up on Whatsapp, which is a low enough barrier to welcome users that usually avoid scary things like creating an account.
And they had extremely wide platform support in times when not everyone had a smartphone. So, like SMS and in contrast to many other apps you were not confined to just Blackberry, iOS, xor Android.
But why is it WhatsApp? It's very frictionless. WhatsApp is a drop-in replacement for SMS. You don't make an account. You just download the app, import your phonebook and start messaging people who are also on WhatsApp. It's SMS except it's free, not more not less. My parents (not from Brazil) are on WhatsApp. They don't use any other social network or messaging app (except Skype).