Hi Glenn! You raise a number of good points and questions, and I'd like to approach our large topic by focussing on your last paragraph and especially your last sentence.
My speculation is this: To the extent that people are using the blogosphere to congregate in groups of like-minded types, they are more likely to be politically active. To the extent that people are hearing lots of different points of view, they are more likely to sit on the sidelines. In short, there is, for many people, a tradeoff between a) exposure to diverse views and b) political participation.
Intuition helps explain the possibility of such a tradeoff. Like-minded people, talking only with each other, get more confident and more extreme. (This is the concern of Republic.com 2.0.) If people are living or communicating in echo chambers, there will ultimately be a kind of Babel effect, in which different groups won't quite understand each other. (Cf. the debate over climate change.)
The bad side of all this includes greater extremism, more anger, and less understanding of one's fellow citizens. The good side is that if you are confident and charged up, you're more likely to be active.
So much for intuition. Diana Mutz' superb 2006 book, Hearing the Other Side: Deliberative Versus Participatory Democracy, offers a lot of evidence, and is full of implications for the blogosphere. If you deliberate with a wide range of people, you are less likely to be engaged in politics (apparently because you conclude that different views are reasonable, and it might not be worth spending your time fighting). If you talk with people who agree with you, you are more likely to get involved. (I bet that Mark Penn knows this; I'm sure that Karl Rove does. Cf. the success of conservative and liberal groups on law school campuses.)
On the blogosphere, some people operate as polarization entrepreneurs. They attempt to create enclaves of like-minded types, intensifying their antecedent convictions. (I won't name names.) The Army of Davids includes a lot of people who have been energized by polarization enterpreneurs -- and as a result, their judgments may have been badly distorted. But it is worth emphasizing that the same processes that create the distortions help to fuel the participation.
As Mutz emphasizes, this benefit is puchased at a cost: Those who have been polarized are not likely to be tolerant of others, or even comprehending, and they might well fail to persuade those who tend to disagree with them. My main point is that to the extent that the blogosphere helps to create information cocoons, it does indeed energize people.
There are obvious questions here about the complex role of two virtues: respect and charity. Any thoughts on any of this? Do we have any disagreements thus far?