Once again GDP has nothing to do with anything here. Whether it was higher, lesser, or whatever else does not matter. Advertising spending increased exponentially only because exponentially more money was being spent on advertising. How exactly that relates to GDP has no relevance, whatsoever.

In 1980 (this study didn't even cover 1970), there were something like 1750 newspapers for something like 60 million readers. So the subscribers per paper was already (relatively) very low. And with classifieds you get a very tiny subset of that that opt-in to reading the classifieds, and further opt-in to the section you are listed in. It should be beyond obvious that this has basically no relationship whatsoever to modern advertising.