Bing had an US  market share of 10.7% in August, according to Nielsen Megaview Search. That positions Bing as the fastest growing search engine in the US top 10. here are the detailed data:

Top 10 Search Providers for August 2009, Ranked by Searches (U.S.)
+------------------------------+-------------+----------+------------+
| Provider                     |  Searches   | M-O-M %  |  Share of  |
|                              |    (000)    |  Growth  |  Searches  |
+------------------------------+-------------+----------+------------+
| Total                        | 10,812,734  |   2.9%   |   100.0%   |
| Google Search                |  6,986,580  |   2.6%   |   64.6%    |
| Yahoo! Search                |  1,726,060  |  -4.2%   |   16.0%    |
| MSN/Windows Live/Bing Search |  1,156,415  |  22.1%   |   10.7%    |
| AOL Search                   |   333,231   |   1.8%   |    3.1%    |
| Ask.com Search               |   186,270   |   2.9%   |    1.7%    |
| My Web Search                |   128,432   |   0.5%   |    1.2%    |
| Comcast Search               |   50,328    |  -21.6%  |    0.5%    |
| Yellow Pages Search          |   37,923    |   2.7%   |    0.4%    |
| NexTag Search                |   31,830    |   0.4%   |    0.3%    |
| Local.com Search             |   16,314    |   2.9%   |    0.2%    |
+------------------------------+-------------+----------+------------+
Source: Nielsen MegaView Search

Google still leads the pack from a comfortable 65% (this is almost a monopoly on search), Yahoo is down 4% above the previous month. It would be useful to compare the Bing market share (taken by the number of searches) with Bing market share taken by the number of searchers. That would allow to draw some interesting conclusions.