I created this thread after seeing an article on New York Times Spanish website today. There is a link at the start of the first paragraph to an English version.
http://www.nytimes.com/es/2016/12/27/wh … migrantes.
I guess the answer to my question may be because when you buy minutes with Movistar you get free minutes with whatsapp. The other day when I put $3 on my account, I got a certain quantity of megs (I think 10 megs that expire in 5 days. So if I was in a non-wifi zone I could call another person who has Whatsapp on their for phone for free.
And for the big money expats with monthly plans with Movistar and maybe Claro, their plans often allow them to make unlimited calls with Whatsapp or unlimited use of Facebook.
Skype also has its advantages, but it is a phone-centric world for many.