SOURCE FOR FOLLOWING DATA:
http://www.faa.gov/data_research/aviation_data_statistics/civil_airmen_statistics/2010/
Total number of pilots (including students, recreational and sport): 627,588
Total number of instrument rated pilots: 318,001 (63%)
Total number of commercial pilots: 123,705
Total number of ATPs: 142,198
Total number of Private pilots: 202,020
Private pilots with instrument ratings: 55,979 (
so roughly a quarter of them)
Sport pilots: 3,682 (
a very, very distinct minority of all pilots at 0.59% of the total pilot population)
Just for the sake of comparison and thoroughness, even if every single sport pilot in the US was an EAA member, that still makes them only 2% of the membership (assuming 160,000 members as stated on the EAA Wikipedia page).