I found this neat podcast on How to find influencers using vertical search engines, social networks like Facebook, Advanced Search in Google (using advanced search will narrow the result set).
You can even narrow your search by when it was found on the Internet (by specifying time) and Technorati is powerful for finding topical blogs that are considered Influential (by the Technorati Score).
The idea is do do a deep dive in to how a company creates Buzz - and you can use Delicious (by tagging bookmarks on Delicious - and look at articles tagged by the number of people.
Flickr, as a photo-tagging site, has no peer - and you can find communities of people who are passionate about a topic - and have taken pictures of those things - and then you can look at who is commenting and what they've been saying, etc.
The issue might be that not everyone is into doing this kind of research which can be fairly time intensive - but there are companies that will help with audience research (but it will be expensive) and will monitor online conversations (some have nice dashboard interfaces).
However closed Social Networks like Facebook are not open to Search Engines. Interesting.
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