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The findings come from a UC Berkeley study, which also found that only four of these top web sites using Flash cookies mention them in their privacy policies. From Wired:
Unlike traditional browser cookies, Flash cookies are relatively unknown to web users, and they are not controlled through the cookie privacy controls in a browser. That means even if a user thinks they have cleared their computer of tracking objects, they most likely have not.
We've shown you how to get around these sorts of "super cookies" in the past, most notably with the previously mentioned BetterPrivacy Firefox extension, but Wired also points readers to the always popular CCleaner and Flush.app, a Flash cookie removal app for OS X.
If you're serious about your online privacy and thought blocking traditional browser cookies was doing the job, you may want to consider trying one of the tools above and hitting up the Wired post for the full rundown.