Google (GOOG) saw its share of “total explicit core search” activity in August in the U.S. decline by three tenths of a percent to 64.8%, research firm Comscore (SCOR) reported this afternoon, while Microsoft (MSFT) saw an equal pickup in its share, rising to 14.7%.

The report follows a similar one from Experian Hitwise last Thursday that revealed a similar trend: a slight decline for Google and a slight gain both for Microsoft’s “Bing” search engine, and for all queries through Bing and Yahoo! (YHOO), which outsources its search to Microsoft.

I would note that the data were released to the brokerages yesterday, so they’ve actually already been all over this.

For example, a note from Citigroup analyst Mark Mahaney from yesterday evening said that the slip for Google was probably a reflection that the bounce the company experienced a year ago with the introduction of its “Google Instant” feature has now faded:

Our interpretation – under comScore’s methodology for Instant, GOOG made initial query share in-roads since its Sep-‘10 Instant launch (share went from 65.4% to a high of 66.6%), but now has now trended back below initial Instant levels. Note that comScore doesn’t include Mobile Search, which is growing rapidly for GOOG.

Google shares today closed down 60 cents at $529.52.