Demand Media is a content farm; they pay people to write articles based on their interpretations of the Google algorithm. 95% "White-hat SEO", and no different from what places like HuffPost does. If the algorithm changes, they can make adjustments, do better keywords and content policing, and still make a buck.
Link farms, on the other hand, camp domain names and make a website entirely of Google ads, keywords and algorithms, usually by exploiting flaws in the rankings system that would normally discriminate against this kind of thing, aka "Black-hat CEO". They don't tend to have IPOs though, because anyone with half a brain knows they won't last once the loophole is fixed.