The problem is, Quantcast is very accurate when it directly measures sites, but less so when it is forced to estimate traffic as it is in this case.
ComScore, which is more accurate when Quantcast does not directly measure, says Pinterest is up from 26 million US visitors in October to 28 million US visitors in December.
NOTE: An earlier version of this post reported only Quantcast's estimate. That was dumb, and not very helpful to readers. We apologize.
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