BJS found that marijuana was involved in the conviction of only 2.7 percent of all state inmates and about 1.6 percent of the state prison population were held for offenses involving just marijuana, while just 0.7 percent were incarcerated with marijuana possession as the only charge.
http://thehive.modbee.com/node/20998
I believe if you checked jails as opposed to prisons (for felonies) my statement is quite accurate.