Troubling news for Microsoft (MSFT). Hitwise’s latest numbers are the second set of metrics from a Web analytics firm showing Bing’s market share in decline. Last week, StatCounter claimed Bing’s share of the U.S. search market in September had slipped to 8.5 percent from 9.6 percent in August.

As I noted then, while a month of slight decline might herald the beginning of a trend, it certainly doesn’t guarantee one–especially in search, where surges and lulls in market share are quite common. That said, this is the second set of data suggesting that Bing’s traffic may be leveling out. Whether this reflects the end of the big Bing marketing campaign or falling consumer interest remains to be seen.

Incidentally, Microsoft’s new search partner, Yahoo (YHOO), also saw its search share slip for the month, according to Hitwise. Yahoo claimed 16.96 percent in August. In September, it claimed 16.38 percent. Meanwhile, Google (GOOG) took 71.08 percent share for the month, up from 70.24 percent in August.

IndexTools “will give our customers tools for monitoring and analyzing Web sites and marketing campaigns, providing valuable insights into key metrics, traffic patterns and performance,” Bassel Ojjeh, head of Yahoo Strategic Data Solutions, wrote in a post to Yahoo Anecdotal. “And that means consumers are more likely to see marketing content that’s engaging and relevant.”

One would hope so. Because at this point, Yahoo doesn’t need to simply distinguish itself from Google (GOOG) Analytics. It needs to catch up to it.