My bet is the majority are private individuals. Because people are more likely to respond to that than a "normal" person. Just look at your sibling comment who said:
> I'm not sure if I'd count a person who can buy enough investment properties to justify using a mass-texting service an "average person like me"
If your average HN reader can't believe an average person can buy investment properties, then likely most Americans won't :-)
Anyone can set up a company in a few minutes. And then pay someone to design a professional looking mailer.
There are RE bootcamps/gurus that walk you through the whole process (which LLC to set up, in what state, which online service to use to make those mailers, which data broker to get the mailing addresses from, etc).
BTW, don't pay for those bootcamps. They are anywhere from 10-100x overpriced.