Feedster's new list of what it calls the 500 top blogs, largely based on the ambient prevalence of links to those sites across the web, spawned several prominent discussions as to how useful blog media may be for advertisers. The list itself was published in part because of encouragement by some bloggers unhappy with the methods used by Technorati to rank its top 100. And the subtleties of these differences underlay the core of the issue of why blog media may or may not prove useful for some advertisers.

Typically, ad agencies like to rank traditional media in terms of popularity, with circulation and ratings providing a surrogate measure for quality and relevance. That instinct is served with the new list, and according to an insightful ClickZ canvassing of the buying industry, some buyers are beginning to cautiously look to the list for this sort of validation.

But marketers using blogs as media fall into one of two very different groups: consumer marketers and business-to-business marketers, and the mechanisms for buying media relevant to one are quite different to those employed to buy the other. Where consumer marketers see a direct correlation between site popularity and its usefulness, b-to-b marketers often find the opposite, as they instead seek highly discrete, specialized audiences. Mixing Boing Boing and Fleshbot with the New England Journal of Medicine and Search Engine Roundtable conflates not just two different types of sites, but also two different types of buying processes.

Feedster chief Scott Rafer notes that his firm never really intended the list to be a planning tool. The fact that ad agency people are looking to it as a resource may be an indication of the lack of useful planning tools in the space. Syndicated research sources such as Nielsen/NetRatings and comScore employ methodologies that work about as well as traditional media metrics for measuring traffic and audiences for the largest sites, but for sites with specialized content and audiences, the data are simply too sparse to inform buying decisions.