This is old data (2002 survey): approximately 57 million people, 27.3 percent of the population age 16 or older, rode a bicycle at least once during the summer of 2002. The survey breaks this down by gender, age, and race/ethnicity. http://www.walkinginfo.org/library/details.cfm?id=14 (survey)
Here’s a 2001 study on travel by all means: http://www.walkinginfo.org/library/details.cfm?id=3185
Here’s another source for surveys more current: http://nhts.ornl.gov/publications.shtml (US DOT Federal Highway Admin.)
Take a look at this policy paper from Rutgers (Puchler, Beuhler, Merom & Bauman). http://www.policy.rutgers.edu/faculty/pucher/2001-2009.pdf The Chart below is from the paper.