According to comScore 66.8% of US mobile users text. Pew found the overall US number to be 72%, while the number of teens who text is 87%. Their SMS average (according to Pew) is 50 per day vs. 10 for adults. And 31% of teens send more than 100 texts a day; 15% send more than 200 a day (see bottom chart below).
Nielsen has now come out with some new data on mobile usage that focuses on texting by demographic segment:
Text messaging is now the top teen motivation for getting a phone.
Voice usage vs. SMS has actually declined. The chart immediately below shows the number of minutes per month of voice usage by age group. According to Nielsen, "voice activity has decreased 14 percent among teens, who average 646 minutes talking on the phone per month."
In contrast to voice, mobile Internet and app usage is growing among teens: 49% of mobile teens accessed the Internet in Q2 vs. 40% last year.
Finally here's earlier 2010 Pew data on the daily volume of SMS messages in the US by age category:
I think we can say without question that SMS is the top "medium" for teens in the US. It may in fact be the only channel that commands their attention to the degree any single medium can now.