I've never seen a person in the US actually use WhatsApp to talk to another person in the US. I used to occasionally see someone use it to talk to friends who were traveling abroad, but that's mostly been replaced by iMessage or Hangouts at this point.
WhatsApp is huge outside of the US. If memory serves correctly, SMS has always been expensive and MMS never really took off - once the data messaging platform came to be WhatsApp really served that market and has done well because of it.
I've paid for WhatsApp and am in the US. Yes, I originally joined to chat with people overseas, but now it's also the sole messaging service I use with multiple US-based contacts.
AMA I guess, since apparently fellow Americans don't understand WhatsApp.
Ya I never really used WhatsApp but I do use Telegram for my friends (US). Why Telegram over WhatsApp or Kik or whatever? Desktop client, it is nice to be able to reply without pulling out my phone at work.
Replace "understand" with "care about" and it's a double win - you're closer to the truth, and you aren't condescending in service of defending your favored corporation.
Outside of the US, not everyone has a phone capable of running iMessage or Hangouts. WhatsApp happily runs on feature-phones, because of network effects, many people who can run iMessage/Hangouts and WhatsApp
