Topic: How do I find key influencers within online communities?

I want to identify who the key influencers within online communities. Are there any applications or tools I can download / use to do this.

For example, Twitalizer gives statistics on users and their influence, which is great. However, it does not list all the users' statistics, only a few.

For example, if I want to find the key influencers within a certain subject matter, such as

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Influencer Marketing Switzerland