Viber is also huge in the Philippines as a telecom there offers free Facebook + Viber without needing prepaid credit/wifi. They could roll a textsecure layer too
WhatsApp is also owned by facebook, so.. i doubt they cut of ways to get to the communications. I think it might use textsecures encryption but they will also have key.
Sorry but my experience is the opposite. Here in France most mobile phone contracts give you unlimited sms + mms. I never felt the need for whatsapp personally, neither do I know anyone using it. (but Viber over wifi is often used by people traveling/working abroad)
No need to be sorry, data > anecdotes. I use Whatsapp with a group of 20 friends to organize nights out and basically use it as a long-running IRC room. We also have different groups for movies, board games, etc. It's the group functionality that makes it better than SMS.
It's very popular in Europe, even more so (as far as I've heard) in South America, where it's used mainly instead of SMS. Heck, even my father use WhatsApp and I can't make him stop clicking on "You have 1 new message" ads. Though I deleted my WhatsApp as soon as it was acquired by Facebook.