That's not a replacement from SMS since those aren't based on phone numbers. If you have the phone number of somebody and they have WhatsApp installed you can m

That's not a replacement from SMS since those aren't based on phone numbers. If you have the phone number of somebody and they have WhatsApp installed you can message them immediately. If you both have AIM or MSN installed you have to exchange your handles through some other channel. That's a massive difference in terms of usability and friction to get started.