Here's a chart that shows Google's US market share in the first half of this year, as measured by the same source.
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We'll see next month if this is a trend, and Google's traffic acquisition deals aren't effective, but, most likely, this is just a transitory decrement in popularity.
Update: Nielsen / Netratings says that Google's US market share in July was 49.2%, down from 49.4% in June.