If we take out spam, is SMS usage growing as much or as fast as BulkySMS believes? I guess not - caution, theory not tested.
Taking the USA market and comparing it to other markets like South Africa will not provide realistic assessment of the markets which have different variables. Like what others have already pointed out, the cost of sending SMS in USA is almost next to zero. In our local market it costs about $0.20 plus per SMS. There are more prepaid subscribers in SA than contact subscribers (75% vs. 25%) but the opposite holds for USA. Since SMS' are in many cases bundled within the contract packages, there is usually no reason for someone to check his/her remaining balance after sending an SMS, which may be the case for prepaid services. With IM as an alternative solution, it would be interesting to compared the number of SMS send by people with smartphones against the number of IM sent by the same people per day. You might be surprised with the outcome.