Right-wing and left-wing partisans alike have blogs they travel to every day for their daily screechifying, such as Instapundit and Daily Kos. But do these sites have any effect? And who’s winning the war of ideas — and producing more real-world influence — in the blogosphere?

A new report by NDN PAC, a progressive group, has been the first to try and quantify the impact of both conservative and liberal blogs. The results are surprising: They conclude that conservatives had an early headstart, such that in 2003 the right-wing blogosphere was two or three times larger than the liberal one. But by 2005, progressive blog world had erupted in size — triggered by the 2004 elections — and is now double the size of the conservative one, in terms of sheer traffic.

Yet NDN concludes that liberal blogs are not necessarily having as big an impact, because of fundamental differences between the way conservatives and liberals use blogs. Liberals may have more traffic, but they have fewer overall blogs. To put it another way, progressives have a small number of enormously-well-read blogs, which conservatives have a large number of blogs with small audiences. That’s partly because of how conservatives use them: NDN claims that the right mostly uses blogs as extensions of pre-existing party structures and organizations; they also more often devote blogs to local issues. The upshot is that conservative blogs have a bigger impact on the real world, since they’re connected to real-world party structures and are focussed on real-world problems all over the country. As the report writers note:

Pennsylvania offers a useful case study. Philadelphia is arguably the nation’s progressive blogging capital. With at least fifteen of the one-hundred and three progressive blogs surveyed by MyDD, not to mention ten of the top fifty most trafficked left wing blogs, one might imagine that local Pennsylvania political blogs are dominated by progressives. Yet, the primary two sites dedicated to Pennsylvania statewide politics were Grassroots PA and Keystone politics, both of which are conservative. Even in a region steeped in popular left wing blogs, conservatives rule the local political blogosphere.

It’s a really interesting report — I’ve never seen anyone analyse cyberspace this way.