This statistic is, ironically, based on a rather poor reading of the NALS report from which it's drawn. The real number is significantly lower.
See, for example,:
http://archive.ala.org/alonline/crawford/cf602.html
http://nces.ed.gov/naal/resources/92results.asp
Hopefully the updated version of the study, due in May of this year, will provide clearer data on which to base conclusions.
-- Matthew Snyder Philadelphia, PA