Earlier quoted context omitted.
We hear this rant on every chat-related thread. WhatsApp brought the amazing innovation of using your phone number for "login" (and later on - your native contacts as its contacts list). This enabled zero-BS onboarding, and that's why they have so many users. Thanks WA for helping humanity deprecate that horrible XMPP. I don't miss it.
People wanted free text messaging, so they used whatsapp. If they were so great it would have caught on in the US, but it didn’t.
Back on topic, I've avoided using WhatsApp for about a year, even though everyone around me was using it. When I've finally gave in I found out that it was really, really easy to use, even on my relatively crappy iPhone4 (I'm talking about ~2014-2015, at which point that type of phone was seen as a museum artifact; I'm still using it). Sending photos was damn easy, notifications were always "right" (not the crappy shit that Skype calls "notifications") and the "last seen" future was the central point of many personal relationships (still is). Unfortunately WhatsApp stopped functioning for me about one year and a half ago (after the FB acquisition), but you can't dismiss it just because it hasn't caught up in the States.