What I really like from WhatsApp/Telegram/Apple's Messages is their simplicity, no over-engineered app for msging, an app that even my dad can use. I want to use the app that any of family members can use.
Unlike, for argument sake, Path, which requires me to register with them, makes me use a "complex" menu, unnecessary status wall, tell me how many connections I should have, etc.
My ex didn't have a smartphone but her phone had WhatsApp and Facebook and pretty much nothing else from the smartphone world was supported. I don't think that I had any choice other than WhatsApp since the person I wanted to talk had no other choice.
I was using it not because of the interface or anything but because it enabled me, I personally don't like the visual design of WhatsApp btw.
WhatsApp become incredibly popular here in Turkey and I suspect that it's mostly because of the fact that it was the only messaging app that almost everybody can install to their phone, including non-smartphone users. Everybody here have at least one phone but not everybody had a smartphone and if some of your friends are out of reach, its a deal breaker.
Line, I belive had little bit traction, Viber too. WeChat even run TV ads but eventually, everybody have WhatsApp :)
Unlike, for argument sake, Path, which requires me to register with them, makes me use a "complex" menu, unnecessary status wall, tell me how many connections I should have, etc.