There is no way to track "bookmark" traffic. Tracking the favicon hits only tracks for IE which these days can be as low as 80% of your surfers.. Just tracking no referers also counts crawlers,bots,spiders,anon surfers and bookmarkers. You can take the norefs, look at your favicon hits, then check out how many people search for your site..mix it all up and come up with a rough idea.. but why does it matter? At the end of the day, if you're getting 20k noref on one site and 100k noref on another, you KNOW you've got a shitload more bookmarkers on the second one.
But if you really need some number, I've found that 60%-80% of norefs gives you a good rough estimate of bookmark traffic. But your mileage may vary.
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