The number of Californians who walk, bike, or take transit on an average day has doubled in the last decade, according to a new study by Caltrans. Also notable: More people in their 20s and 30s are not getting their licenses at all. [Los Angeles Times]
Another thing to be considered with Californians not getting their licenses —illegal immigrant population. One out of ten residents in Los Angeles is an illegal immigrant. Many drive without licenses, or do not drive at all. I think other counties in the state have similar figures, which would affect the number of counted heads vs the number of licenses issued.
Source: USC, as quoted by LA Times
I agree with the linked articles main underlying point though... it's simply just too expensive to drive as much as before.