I don't find there is much network effect for one on one messaging. I have to use a few different apps to talk to all my friends, it's not a big deal to switch to/from Signal or Whatsapp. With groups it's more effort.
This does not match my experience in Germany. If somebody gives you their phone number it is just expected that you can reach them on WhatsApp and i have yet to meet anyone that doesn't use WhatsApp.
That seems true throughout the most of the Western world, excluding the US. I have a big WhatsApp network, but that's by virtue of living in SF and NY. Without big immigrant/expat/world-traveler communities, I think most of the US just uses iMessage or regular text.
