Wikipedia puts the fraction of the U.S. population using the internet at about 8...

Wikipedia puts the fraction of the U.S. population using the internet at about 81%.[1] They link to a source, but beware it links straight to an excel file[2].

[1] http://en.wikipedia.org/wiki/Internet_in_the_United_States#A...

[2] http://www.itu.int/en/ITU-D/Statistics/Documents/statistics/...