An interesting graphic in the Atlantic charts the dramatic decline in print advertising from the late 1990s to 2011.
The accompanying article argues that this helps to explain the decline in newspaper circulation, since papers rely on a cross-subsidy from ads in the car/style/travel sections and the classifieds to pay for their major news coverage. But ad dollars started flowing to websites that gave consumers their car, style, travel, or classified ads directly, and the readers followed.
ABC circulation data has shown that, in the UK, newspaper circulation has been falling steadily since the late 1980s, long predating mass take-up of the internet. Clearly there must be additional drivers to this decline; perhaps there is a cohort effect at play, where younger generations are just not reading newspapers in the numbers they once were.
I would expect print advertising to decline further as new technologies, e.g., tablet computers, provide alternative platforms for advertising spending.