Congrats! on your panel. Glad to see people addressing this issue and especially at a (traditional) blog focused conference. Clues me in that people are taking blogs more seriously as a credible marketing strategy. As you indicated the first step to defining what to measure is to identify what is success. Facebook is looking at what they call "Extent of sharing." How many times people share within a community. We are in for an interesting ride as new "success" objectives are identified. Now to develop the tools to do the job!