Much as the development of agriculture
marked humanity's discovery
that it was less work to just trick plants into growing next your house than to run out into the forest looking for them every day, the advertising
industry marks the discovery that it is easier to manipulate
people into buying
what you're selling
, rather than relying on them to buy it on their own.
The existence and open acknowledgement of this industry, in addition to its working premise and nomenclature (i.e. "markets", made up of "consumers"), are a tell-tale indicator of the boundaries of the modern two-caste society. This, roughly, consists of producers (business-owning aristocrats or politicians) and consumers, who are, given the advertising industry's gross revenues in 1999 (~$215 billion in the US alone), effectively the biggest cash crop since the invention of agriculture.
The "mass-media" and other 20th century advances in communication technology have revolutionized advertising, having the effect of creating tremendous economies of scale to benefit the producer class, and causing the single largest reenlargement of the class gap since the widespread fall of classical monarchy over the previous 200 years.
Required reading: Frederik Pohl and C. M. Kornbluth's, The Merchant's War.