I would bet that the dominant form of advertising of the 21st century hasn't been invented yet.

First off - take a look at the historical technologies he calls out in the post. Radio and TV didn't even exist yet at the height of the Gilded Age. The dominant media was newspapers (Pulitzer & Hearst, etc.), which ran a scaled up, largely self-service, mass-media form of the advertising which worked in the 19th century. Much like how Google/YouTube/Doubleclick, Facebook, and Craigslist run scaled up, personalized, carefully tracked forms of the advertising that worked in the 20th century.

The concept of "brand advertising" at all didn't exist until the early 20th century. Why? Because "brands" as a concept were enabled by the technologies of mass production, mass consumption, and mass media. Before it becomes economical to spend millions of dollars building awareness, need to have an audience of millions of people with the disposable income to buy your product, and you need the capacity to make & distribute millions of them. The capacity to build them became available in the 1870s, the capacity to distribute them in the 1890s and 1900s - but the audience of millions of people who could afford them didn't happen until living wages started to become a thing in 1914, and it took a generation to spread throughout the population.

I'll predict that the 21st century will see a trend to toward smaller firms, hyper-personalization, local manufacturing, and custom services over products, much as described in The Refragmentation [1]. The technologies of cheap solar energy, 3D printing, the Internet, self-driving transportation, and pervasive computing (beyond mobile - the wearable market will dwarf mobile to the same extent that mobile dwarfs PCs) will make it much more economical to build small production runs tailored tightly to individual preferences than mass-market consumer goods. The forms of advertising used to make people aware of these goods will shift as well, creating a new category on top of "brand" that is to brand advertising as brand is to coupons, but who knows what that ultimate form will be? If you can figure it out, it's probably a trillion-dollar market.

I think that Etsy, Kickstarter, Yelp, LinkedIn, AirBnB, etc. are all harbingers of the type of company that will dominate the 21st century, but they're struggling now because they're early. People haven't yet let go of the old ways of thinking of social & economic organization, so they evaluate 21st-century business models in terms of 20th-century industries. Yelp and LinkedIn in particular are in trouble, because their customers are all embedded in old-economy value chains (local businesses, employment), and so they're in this weird position where their own business model assumes a networked, hyper-personalized world but their customers will disappear if that world comes to pass. It'll be interesting to see whether these first-generation marketplaces can hold on and prosper in a few decades, or whether they'll go under and be replaced by companies that build on their ideas but with new value chains. Historically, most of the first-movers in the industrial revolution died out, and the household names of the 20th century were large conglomerates formed by their carcasses.

[1] http://www.paulgraham.com/re.html