it really depends on your type of content and where your best customers can be found.
in the case study, we used Facebook, Twitter, YouTube, and Betternetworker as our main ones. we injected PPC traffic from Facebook, Adwords, and Bing. i think we spent a total $2,000 and generated 30,000 leads.
the main driver of the whole thing was Betternetworker because it had our main target audience (network marketers who wanted to market on Facebook, Twitter, and YouTube).