The real reason Google's clicks are flat. Rich Skrenta explains that Google’s recent reduction of the clicable area in Adsense ads, while reducing click-throughs by 60%, will eventually balance out due to non-accidental click-throughs being worth more to advertisers.
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Wow, I hadn't even noticed the change. This is nice, though, because it's nice to be able to click on the ad copy without being whisked away.