What app you americans use for personal conversations?
Here in South Europe people mostly use Viber.
Edit: I was very unaware about situation in Southern Europe as I've learned from this post...
Most people in Croatia use Viber!
I just use SMS. I don't need to install anything extra or make some sort of extra account somewhere. Phone plans here have had unlimited texts for like over a decade now so we aren't spending extra money here to send texts either.
SMS is notoriously unreliable, plus insecure as hell.
As in it's known across the industry that upward of 10% of messages fail, and since SMS lacks error detection (and thereby no error correction), you have no idea when your message never arrived.