Alexa, Digg, Reddit, Delicious: How Accurate Their Estimates?

More on the inaccuracy of traffic estimation services: Paul Graham examines numbers from Alexa, Digg, Reddit, Delicious. Graham concludes:
If anyone wants to start a startup that will somehow yield reliable traffic numbers, we'd love to fund that one.
Update: Relevant post from SiliconBeat from August on the impact of inaccurate traffic #s on sites, esp. startups
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