According to the New York Times, a recent poll performed by Ipsos says that, "82 percent of cellphone owners said that they never used text messaging, 3 percent said that they used it monthly or less, and 15 percent said that they used it every week or even more." In other words just 15%, or 45 million people, are contributing the vast majority of the text messages being sent each month.
According to CTIA, The International Wireless Association for the Wireless Telecommunications Industry, text message sending and receiving jumped from 14.4 million in 2000 to 363 billion in 2007, with a monthly average of about 48 billion. In the first quarter of 2008, Verizon Wireless alone delivered nearly 58 billion text messages from coast to coast.
Looking at 2000 census data, ~20% of the then-population of the U.S. were aged 13 to 19. The numbers are surely a bit different today, but they can't be that far off. So all those huge text messaging numbers that the network operators are crowing about each month? Yep, it has to be all from teenagers. Seriously.
What population segment Access and Amplitude spoke to about messaging features had to be skewed heavily towards power users who actually do more than stick their cell phones in their glove compartment for emergencies.