The impact of fake/spam blogs

In a free speech society, who makes the decision that a blog, or any Web page, is "fake," or spam? Dave Sifry exudes confidence in his assessment. But I can easily see some gray areas to be exploited between a blog like this and journals such as mine. Who decides? The Blogger terms of service don't prohibit such content. The big challenge will be to search engines, to filter out what the general public perceives to be fake content, or spam. Could this turn into a slippery slope though? How long will it be before someone's authentic content is inadvertently tossed aside in such search engine filters?