I thought this was funny when I went to work in Korea for a year, as an American, with a bunch of other Americans; all of us stationed there in the military. Upon arrival, the other Americans immediately told me: "You need to get Kakao. It's how you text in Korea." I complied, but I often made the point that it was just another mobile app that they could install and use even back in the United States, and something like Hangouts would work just as well in Korea. When I moved back to the United States, I kept using Kakao to stay in touch with some of my old co-workers who were still in Korea, and this blew their minds, including some people who were otherwise quite tech savvy.

Note that I don't think many of us were using Kakao to talk with native Koreans, which would be a good reason for using it. Honestly, we didn't mix much. Nearly everybody on my contact list was an American.

It's similar in Japan. Everyone here uses LINE, and AFAICT, absolutely no one does SMS texting any more. I don't even know the phone numbers of any of my friends here; everyone just exchanges LINE contact info by QR code and that's it. The only time anyone uses a phone number seems to be for non-personal purposes (e.g., calling a business or government office), and the only time SMS seems to get used is for automated messages from official places (like your utility companies).

A lot of people here have iPhones, but it's not because of some stupid blue bubble.

What's funny is that Americans will sometimes make fun of Japan because some people (mainly businesses, maybe some elderly people) still use FAX, but Americans are the ones who are all still using SMS.

I can relate - I also spend 4 years living and working in Korea. And I also still have Kakao group chats filled with just other expats including gyopo, in addition to the ones with native friends. :)

Kakao is in many ways kind of the worst of chats - phone number-bound, unspec'd, closed, non-federated, unencrypted, known to be under surveillance, ad-infested. But on the other hand around 2010-2015 the UX was quite ahead of many others, and while Telegram & co have since pulled ahead it's still nicer than, say, WhatsApp or Signal.

I was using it in 2014-2015, and yeah I thought the UX was fine at that time. I did have the foresight to tie my account to my US Google Voice number (which has long been my primary number), so I didn't lose the account when I returned to the US and lost my KT phone number. I think that's what tripped up the other Americans who thought it was only an option in Korea.