Seemly Whatsapp is, and what they are doing is genius, I would pay even more for the service than what they are charging (99 cents year).

At first I did not understood, what it had in special, until I figured that it was the fact that it used your phone contact list, thus taking out the boring task of managing that, and making adding people to your list easy as finding someone phone number.

Yet, I would not use it as my primary messenger (neither Facebook), because of how they are tied to something in particular (phones, Facebook) and thus allow less flexibility in where or how I use them.