The winner of the LG US National Texting Championships is 14 year old Kate Moore from Des Moines Iowa. She sends about 14,000 SMS messages per month and up to 470 per day. (I hope she has an unlimited messaging plan.) She won a $50K prize in a head to head competition for speed and accuracy after multiple elimination rounds. 

US teens and young adults text more than they talk on their mobile phones. According to Nielsen (data Q2 '08): 

Americans 13 to 17 years of age sent or received an average of 1,742 text messages a month in Q2 per Nielsen. SMS penetration is 53 percent, or 137.8 million mobile subscribers in the US.

Many people regard SMS as a kind of mobile social network. At first blush that's strange but it makes sense if you reflect.

The following are LMS/Opus online consumer survey data from early 2008. Mobile access to social networks has grown since that time. But the chart illustrates the way that some people regard SMS as a kind of substitute for social networks on mobile devices:

Source: LMS/Opus (3/08), users who do not access social networks on mobile devices (n=730). Total survey sample 1,022. 

Longer term, it's possible that mobile IM and communication via social networks over mobile devices could cut into SMS volumes.