Some things should be obvious but evidently they are not. The PENETRATION of WhatsApp is way ahead that of Viber and Skype.
Comparing WhatsApp to Viber and Skype is like comparing Google to DuckDuckGo, GigaBlast, Yandex etc. I am sure most Wazuans have never heard of DuckDuckGo, GigaBlast and Yandex. Guess what they are going to do. Google them! Just because something is AVAILABLE and is not being used doesn't mean other products offering the same services will face the same fate. There are very many factors that affect uptake.
Ever heard of cuil??? Probably not. It was a search engine started, run and managed by former senior Google employees. Guess what happened to them. The company died a natural death! Guess what has been happening to Google! Just because people totally ignored cuil it doesn't mean that Google is under the same risk.
Back to WhatsApp. It is prevalent among smart phone users. The company never marketed itself to them! Their friends, relatives etc introduced them to the App or they themselves looked for this info. The App is that popular! Second. Simplicity. A simple call icon at the top and that's it! Contacts tab at the top. All your contacts are there! Install WhatsApp for your grandmother and she will be making WhatsApp calls to you with no additional effort!!!
Never count on making a good sale. Have the purchase price be so attractive that even a mediocre sale gives good returns.