I think the reason that SMS/iMessage has hung on in the US is that they got "unlimited texting" plans early on, whereas in Europe carriers kept charging per-mes

I think the reason that SMS/iMessage has hung on in the US is that they got "unlimited texting" plans early on, whereas in Europe carriers kept charging per-message much longer, driving people to messaging apps simply to save money. Europeans are also more likely to have international friends to send messages to, which costs even more (you see the same thing in diaspora in the US). Before WhatsApp, I remember Skype was very popular as a messenger since people already used it for free international calls. Japan is interesting because their feature phones used internet e-mail instead of SMS, but I don't ever recall seeing a client that threaded e-mail conversations like SMS clients ended up doing.