Microsoft researchers have been looking into the problem of junk web pages, and have found that most of this 'search engine spam' comes from just two web hosting firms.
Microsoft's research, entitled Spam Double-Funnel: Connecting Web Spammers with Advertisers, looked primarily into redirection spam and reveals that the links to spam web pages are generated by a small number of people, with some major advertisers, hosting services and ad syndicators turning a blind eye.