It depends on how you define the market. Advertising as a whole has grown no faster than GDP over the last century, yet specific sectors have grown much faster

It depends on how you define the market. Advertising as a whole has grown no faster than GDP over the last century, yet specific sectors have grown much faster over that time (at the expense of other sectors.) Twenty years ago online advertising started to crowd out newspaper advertising, and hundreds of companies (mainly ad networks: I have a list of >300 ad networks from about ten years ago somewhere in my archives, and that was just a sample) sprang up to deal with that demand. Now programmatic advertising is growing much faster than the market, and crowding out ad networks. Advertising was a mature market when Google started. Media was a mature market when Facebook started. (And, to make a slightly different point, people believed computers were a mature market when Apple started.)