SMS:ing is bigger than (mobile) phone calls in Sweden, especially among young people — all they do is text each other, back and forth, even when calling might be cheaper. It's been this way for years.
Key reasons: receiving an SMS is free (I believe this to be true of most countries except the US).
Sending SMS as well as making calls is much much cheaper.
SMS are usually delivered the moment they are sent.
I can only tell you that SMS is big in India. And when I say "big", I mean so mind-bogglingly, utterly huge that you're going to have to grow an extra brain just to imagine the sheer volume of SMSs Indians send each day.
Anyone happen to have stats to support that, I find it very hard to believe based on (admittedly anecdotal) evidence.