NEW YORK (CNNMoney.com) — Once the world’s online search leader, Yahoo’s share has sharply declined, putting it in danger of losing its relevance in a market increasingly dominated by Google.

Yahoo’s search market share in November fell to 17.5% from 18% in October, according to a monthly comScore report released late Wednesday. It’s the lowest share ever recorded for Yahoo (YHOO, Fortune 500).

Cannibalizing Yahoo’s market share is Microsoft (MSFT, Fortune 500), whose new Bing search site gained 0.4 points of the search market to 10.3% in November. That was the first time Microsoft owned more than 10% of the market since September 2007.

Despite that good news, it’s really a mixed blessing of sorts for Microsoft, which entered into a search deal with Yahoo that is expected to start in the next several months. When the deal was announced in July, analysts largely praised the marriage, since the companies held a combined 28% of the market — close to the 30% that experts say is needed to convince advertisers that a company is a relevant competitor in a marketplace.

Since the July announcement, “Microhoo” has gone in the wrong direction. The companies’ combined share has taken a 0.4-point hit, as Yahoo’s share has fallen by 1.8 points, outpacing Bing’s 1.4-point gain.

“They’re still going to be a viable No. 2 behind Google, but less so than they expected,” said Daniel Ruby, research director at search-advertising firm Chitika, Inc. “Everyone is surprised by the fact that Yahoo has lost such a significant amount of traffic. Thirty percent seems like a very long shot.”

Google (GOOG, Fortune 500) grew its share by 0.9 points since July to take 65.6% of the search market in November. That’s the largest share Google has ever garnered.

Meanwhile, Yahoo has lost share for 10-straight months. As the closing date nears for the search rivals’ deal, some say Yahoo is reaching a tipping point that could make or break the value of its partnership.

Under the 10-year agreement, Microsoft will power the searches that users make on Yahoo.com. In return, Microsoft will pay Yahoo 88% of the revenue it gains from searches on Yahoo’s sites. Yahoo.com and Bing.com will maintain their own branding but search results on Yahoo.com will say “powered by Bing.”

“There is no getting around the fact that the market share trend for Yahoo is absolutely awful,” said Benjamin Schachter, analyst for Broadpoint AmTech. “The Microsoft deal does not guarantee any search revenue, only revenue-per-search levels; therefore, search share and volume are as critical as ever.”

Still, another school of thought says not all is lost for Yahoo.

Both Yahoo and Microsoft have poured millions of dollars into advertising campaigns to get users to come to their Web sites. Yahoo’s new “It’s Y!ou” campaign has been plastered all over billboards and television spots. Microsoft just launched its new highly publicized Bing iPhone App on Tuesday.

As a result, some advertisers believe users who search on those sites are more likely to indulge a sales pitch and therefore are more likely to click on their ads than Google’s users.

“Microsoft and Yahoo offer quality versus quantity,” said Ruby. “The traffic they drive is more valuable than Google’s in some advertisers’ eyes, because their users are going to be delivering higher margins.”

So even as Google continues to gain share at “Microhoo’s” expense, Yahoo and Microsoft live on to fight for high-quality searchers as a way to stay relevant.