Perhaps it's just schematics, but your original statement was that one was widely used and the other wasn't. I disagree with that. If you want to clarify that they're are both "widely used" but to different degrees, that's much more accurate, in my opinion anyway.
Here's a more recent study, by the way, which says 52% of Americans over 18 have tried marijuana, and of those, 44% use it now. 44% of 52% is almost 23% of Americans, by the way, which is more of an accurate comparison to your alcohol stats.
New Poll Finds Majority of Americans Have Smoked Marijuana - NBC News