Is there any literature on this? I always figured Google and Facebook dominated the market so heavily everyone else was left with scraps (twitter, for example).
They get the lion's share of attention because they're the biggest players, but that doesn't mean they're the only ones, or even that they own a majority of the market. People in general are not well-equipped to process the idea of a long-tail, where disparate players collectively make up a majority of the market but aren't identifiable as a single organization. By the numbers, there's room for about 400 $400M companies in the space of the online advertising market not claimed by Google and Facebook, or alternatively, for 1.6M sole proprietors who each earn a six-figure income on ad revenue alone.
Twitter, BTW, makes a very respectable $2B+/year in revenue. Their problem is that their cost structure assumes that they're going to beat Facebook; they're supporting 4000 employees with fat salaries & option packages on that revenue (Facebook, with 8x the revenue, has only 4x the employees). If they fired half their employees they could easily be profitable. And the company in the article probably has no more than a few dozen employees, hence its massive profitability.
[1] http://www.pwc.com/gx/en/industries/entertainment-media/outl...
[2] https://www.emarketer.com/Article/Advertisers-Will-Spend-Nea...
Now...if you control the landing page and conversion funnel, basically delivering paid customers (vs. clicks) to a paying merchant, your value per visit goes up by a factor of 10 - 50. $.25/click vs. $10 - $30 (varies by product).
Now imagine a company that launches dozens of offers with hundreds of variations and plows it's bankroll back into the best (most profitable) version (for years at a time).
Yeah, $400MM is possible.