Every month since its launch, Microsoft’s Bing search engine keeps taking a little bit of market share. In August, Bing gained 0.4 percent to end the month with 9.3 percent of search query volumes in the U.S., according to comScore’s Qsearch estimates. Meanwhile, Google’s share came down 0.1 percent to 64.6 percent and Yahoo/s remained flat at 19.3 percent.

In other words, Bing showed the only significant gain, while everyone else stayed relatively flat. That $100 million marketing campaign must be working, or maybe it’s the improvements Bing is making to the search experience, or maybe it’s both. Whatever it is, it is translating into nearly a half-point market share gain every month for the past three months.

Bing is up a total of 1.3 percent from its launch at the end of May. Yahoo, however, is down 0.8 percent in that same period, so the combined gain is only half a point. But Yahoo has stabilized its share, and if Bing can continue to nibble away at the same rate, Google will have to start to actually worry.

In August, it grew faster than Google for the first time, with a 31.9 percent annual increase in search queries compared to 21.6 percent growth for Google and 16.8 percent for Yahoo. How long can Bing keep it up?

U.S. Core Search Share, August 2009 (Source: comScore qSearch)

(Table below via JPMorgan analyst Imran Khan)