Nearly everybody uses it in the USA. It's very common to text people through the default messaging app on your phone there (probably an outgrowth of the popularity of iPhones and iMessage), and backup options would be Instagram, Snapchat for a certain demographic, or Facebook messages. Whatsapp is only for messaging foreigners, and Signal/Telegram are for messaging your drug dealer.

Yep, I have never used anything other than the stock messaging app that came with the phone. Whether that's SMS, iMessage, or whatever has never crossed my mind

I was really disappointed when Signal went away from being used as an "everything" messenger, including regular SMS. I onboarded all my family and now because it's not a default messenger, nobody is using it.