It's actually roughly 85% of people in the US smoke weed at some point in their lives, as you can see here on p. 103:
http://www.monitoringthefuture.org/pubs/monographs/vol2_2009...
(You need to adjust upwards a couple of points to account for those who don't make it to 11th grade, who are assumed to use drugs at 50% higher rates, and also to account for a couple other factors that are discussed in the methodology.)