I'm very much like you. There are several apps that I use to communicate with a handful (or less) of people in the world. I see people on travel sites saying "Just use WhatsApp" and I'm more or less, yes it's installed on my phone and I use it with a couple people but it's certainly not something that most people I know use.
Probably something of a demographic (geography/age) thing.
WhatsApp is very much a geography thing; it's pretty ubiquitous in many countries but relatively unknown in the US.
I think that's probably true although I've had even US people argue with me that "everyone they know" uses WhatsApp. I do think it's the case that SMS became basically free in the US sooner than other places and therefore it became the default in the US whatever encryption or other details.