Aside from a couple non-US friends, I know no one in the US who uses anything other than straight SMS (and Apple iMessage). I'm sure they exist but certainly not in the circle of people I communicate with.
For whatever reason, chat seems to definitely encourage tribalism. The last company I worked for eventually bought into Slack because so many people WOULD NOT use anything else while a lot of us were like "ANOTHER chat app??" because we were perfectly happy with Gchat which we had as part of Google Workplace.
I know there are some historical reasons for non-SMS because of text pricing outside the US but everyone I know in the US would look at you funny if you wanted to use some special app for texting.
There's definitely different circles in the US. My circle of friends and family is on Whatsapp. More than 99% of my communications would be through WhatsApp.
iMessage is very much a US thing. Most of the Non US people or people with international connection exclusively use messaging App ( whatsapp, Telegram, Signal)
Not all my friends switched, I had one good friend who decided not to because she already had a bunch of apps and didn't just want to talk to me on yet another app.
It's much easier when it's a group. I got some of my family to get on it too and they pretty much exclusively use it to talk to me.
In the mid 2010s it wasn't that hard of a call because the various Google apps kept getting deprecated (we were all in hangouts before), iPhone users wanted something rcs like and they couldn't for android users with mms, in general the app scene was taking off with Snapchat wechat etc. so people were easier to convince to dl it.
My pitch was 'you know how randomly Facebook or YouTube will serve you some adds about something you were talking about about, even though you didn't search with them? You're much less likely to have that happen with signal'
Then if they pressed I'd share a link from the net neutrality fight days about DNS hijacking etc and having them remember when all their failed urls would go to an ISP run search domain
I definitely used some FUD but it worked.
Actually I think some of the FUD was 'what if the carrier gets hacked?'.... Which, I mean for all carriers and all systems is just a matter of time. As t-> inf the probability of a breach converges to 1.
Also if any of your friends do drugs, of any sort, that was a great motivator for them to switch lol. Weed has only been legal for recreational since 2013 in any state.
Oh, and pretty much every techie friend I had went 'yo that's awesome' and changed over, even if they don't have a tech job.
Finally, back in the day/for many years, signal could default to normal MMS messaging, so the pitch was 'if they don't have signal, you can just text like normal'