Digby has an excellent article comparing the right-wing vs the left-wing blogospheres.  The thrust?

The right blogosphere operates largely as part of the greater Republican message machine.

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By contrast, the left blogosphere is populated by “citizen bloggers,” who work in non-political occupations for a living and blog for reasons of personal interest. This sphere actually operates as a unique and potentially powerful political constituency rather than a part of the Democratic Party apparatus.