Microsoft’s Bing has stolen a bit of search share from Google (NSDQ: GOOG) and Yahoo (NSDQ: YHOO) in the last week—-  its share of search-results pages increased from 9.1 percent to 11.1 percent since its launch, according to comScore (NSDQ: SCOR). But will the good times last?  Probably not, says Barclays Capital analyst Doug Anmuth. In a report this morning, Anmuth says that while he likes Bing, its share will fall back for the following reasons:

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—The halo effect: Anmuth cites the “halo effect” from other Google products, like Gmail, Maps and iGoogle as an advantage it has over Microsoft (NSDQ: MSFT) in retaining users since people go to Google for many different products.
—History: Anmuth points out that a short-term pop does not always mean a long-term increase in share. For example, he points out that Live Search, launched in 2007, saw its share increase to 13.2 percent from 10.3 percent following its launch, only to fall back to original levels. Live Search Cashback, launched in 2008 had a similar experience—its share rose to 9.2 percent from 8.5 percent only to fall back to 8.4 percent.
—Product differentiation: Anmuth concedes that the difference are small between the search engines, but that Google still has a better index and displays search results quicker. He points to the following URL as an example: http://www.blackdog.ie/google-bing/.
—Advertising: Anmuth estimates Microsoft has spent $80 million to $100 million advertising Bing, and says that as it retreats from those spending levels, Microsoft will have to grow on the strength of its product versus Google’s.

As for the inevitable question of how this will impact any Microsoft/Yahoo partnership, Anmuth still believes one is unlikely given comments by both company’s management teams, but the likelihood of a deal could increase if Bing steals significant share from Yahoo over the next six to 12 months.