At least for the illegal stuff. Prescription drug abuse is up, however. Maybe this is one of the side effects of increasing access to health insurance.
And let's not forget those states--Illinois just joined this group--where medical marijuana has been licensed. My experience in Colorado is that legalization of medical marijuana leads to a large increase in the number of people suffering from pain that can only be "managed" with cannabis. Fortunately, employers in states where marijuana has been legalized in one form or another may still discipline employees who show up with dope at detectable levels in their blood or urine. But as the article notes, the landscape for disciplining employees for off-work marijuana use is changing, as the recreational possession laws start to take effect.
And, of course, there is still the significant conflict between state law that legalizes possession and use of small amounts of marijuana, and federal law, which strictly prohibits any use or possession of the drug. So if you want to smoke dope in Colorado (which recently legalized recreational dope use), stay unimpaired long enough to make sure you're doing so in a state park, and not on a federal national forest or park.