Yes and I would also add that using the phone number / imei as account identifier helped them a lot to convince everybody and their mother to use the service. P

Yes and I would also add that using the phone number / imei as account identifier helped them a lot to convince everybody and their mother to use the service. Personally I'm not a friend of WhatsApp, but the reason even older, far from tech-savvy people are using it is the ease of access - you don't need anyones username or email - just install the app and your existing contacts show up. I've seen people who couldn't remember their own email using the app within seconds on a feature phone. Try that with Facebook.