I use WhatsApp. Two reasons. One, the message seamlessly integrates with insert photos, audio, and video. Traditional SMS does not support those media well. Two, it is very popular out of the US. I have a lot of personal contacts from Taiwan, Hong Kong, and Europe. It is nice to have everyone on the same platform. I have no idea about how much international SMS costs.
Recently (last two years), I have noticed that most of the messaging activities between my contacts and I have be gradually shifting from Whatsapp to, ironically, Facebook messenger. I paid 3 years license a few years back. It looks like I will not be renewing.