traffic is a mindset.
think about grocery shopping. you can pick up a can of coke at a convenience store, at a walmart, a wholesaler, or direct from coca cola (u know what i mean)
I think the companies you listed are just the middlemen. Chances are they are reselling other people's traffic or they generated some from the different facets of internet mediums (that is social, mobile, display and search). I dont know how legit they are, but the BIG clients stay away from shady sources. (JC penny got f-ed when they did
seo spam, google that for a good case study)
here are some things you should search for to learn more about the legit industry:
real time bidding
demand side platform
ad exchanges
adexchanger
multi channel attribution
remarketing
Different channels have different values. People that do email marketing can tell you that their traffic converts better than any PPV buy's or social media content. I only write this because cheap traffic has a value.
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