
From: http://www.emarketer.com/blog/index.php/mobile-advertising-more-effective-online-ads/
Here is the number of SMS messages sent in the month of December in US over the past six years:
2003 : 2.1 billion
2004 : 4.7 billion
2005 : 9.8 billion
2006 : 18.7 billion
2007 : 48.1 billion
2008 : 110.4 billion
At the same time, the number of mobile phone subscribers has risen 70% from 159 million to 270 million in these six years.