Yes, it would seem that sms is nearing its grave, and the networks know it. In fact, if you look at the moment some providers are giving unlimited text (Telecel did this through their Mo' Fire promo, Econet does it through etxt - well there's a little work around here, but it can be done).
MNOs know it and hence are trying to milk the last few cents that they can out of it.
IM from apps like Facebook, Twitter, biNu and WhatsApp have proved a threat to their revenue stream and negatively affected their ARPU.