Earlier quoted context omitted.
I thought it was the de facto messaging app across the world? Is there any particular reason why it is not used much in US?
By the time smartphones were widely adopted in the US, SMS already had massive market penetration. Trying to adopt something else when your entire social network already uses SMS is a losing game. This is why, IMO, Apple, Google and Facebook are the only companies really in a position to change the messaging game in the US. There is space for newcomers (see: Snapchat), but they're going to have to work hard to spread…
I lived in the US for a while, and I can't really come up with any reason why it should be different there. Only that way more people are on Facebook (and those are also way more active). Still, I can't really explain why texts stuck around even for smartphone users.