That's not always true though. Although WhatsApp is very popular here in South Africa, I use iMessage most of the time because most friends/colleagues/family have iPhones.
And honestly I have switched off notifications on WhatsApp (I cannot recommend this highly enough!) so that's the only quick way to reach me. I am sure others do the same.
Indeed. Of course it has to be pointed out that 70% of the population can barely afford a smartphone, so they get the cheapest ones, which are obviously not going to be an iPhone.