I have WhatsApp installed for two different group chats, Google Chat for another couple of group chats, but apart from that SMS is the standard here (which means opportunistically iMessage / Signal). I'm in Australia, where SMS typically have no per-message cost (the only thing that's charged per use on most mobile plans here is data and international calls).
In Ireland, an SMS used to cost 13c, if I remember correctly.
So when apps like viber, WhatsApp, etc came on the scene, people jumped on them quickly and completely stopped texting.
This was before they even had voice calling.
With voice calling, it is also popular, as you can call someone irrespective of what country they are in and not worry about roaming or international call charges (even though in the EU now we do not have roaming charges anymore).
Some people even use WhatsApp for normal calls over normal cellular calls!
