Also, it was free and works on Android (Very important in South Asia).
Many older people get smartphones just to use whatsapp to communicate with others. I'm not joking. I upgraded my mom to a smartphone against her will so that she can use whatsapp to communicate with friends and family more easily.
cf my comment https://news.ycombinator.com/item?id=7267310 or http://www.whatsapp.com/legal/
My interpretation of this and a couple of other hints - you agree to transfer your info to the USA, anyone not blocked that has your phone number can read your communiques - is that they're saying that a USA TLA [or whoever] are guaranteed to be observing you when you use the service. But that could easily be misinterpretation on my part.
"The Service is controlled and offered by WhatsApp from its facilities in the United States of America. WhatsApp makes no representations that the WhatsApp Service is appropriate or available for use in other locations. Those who access or use the WhatsApp Service from other jurisdictions do so at their own volition and are responsible for compliance with local law."
>"Special Note to International Users
>"The WhatsApp Site and Service are hosted in the United States and are intended for and directed to users in the United States. If you are a user accessing the WhatsApp Site and Service from the European Union, Asia, or any other region with laws or regulations governing personal data collection, use, and disclosure, that differ from United States laws, please be advised that through your continued use of the WhatsApp Site and Service, which are governed by California law, this Privacy Policy, and our Terms of Service, you are transferring your personal information to the United States and you expressly consent to that transfer and consent to be governed by California law for these purposes." //
Indeed checking now, from UK on 21 Feb 2014, http://www.whatsapp.com/legal/ still reads as above.
"intended for and directed to users in the United States"