That is a very good point, even if you don't immediately realize the deeper implications. :)
In India, you generally pay per text message - so if you can convince even 3 of your close friends to start using Whatsapp, you save (say) 20% of your phone bill.
Here in US, if you are going to send more than a few texts a month, you are going to be paying 20 USD. So there is no real monetary incentive (initially) for you and two of your friends to switch to Whatsapp.
In India, you generally pay per text message - so if you can convince even 3 of your close friends to start using Whatsapp, you save (say) 20% of your phone bill.
Here in US, if you are going to send more than a few texts a month, you are going to be paying 20 USD. So there is no real monetary incentive (initially) for you and two of your friends to switch to Whatsapp.