The following commentary comes from an independent investor or market observer as part of TheStreet's guest contributor program, which is separate from the company's news coverage.NEW YORK ( Trefis) -- Microsoft ( MSFT) Bing's search market share in the U.S. is expanding and currently stands at 14.7% according to research firm ComScore, driven by increasing number of searches conducted online particularly through use of mobile phones. But the same cannot be said about its revenue per search, still a cause for concern for Microsoft. Notably Yahoo ( YHOO) and Microsoft entered into a search advertising partnership in 2009 to boost their search revenues. Yahoo's ads are powered by Microsoft's adCenter platform, which has had consistent technical issues and has resulted in lower RPS for both companies. Yahoo's ex-CEO Carol Bartz has also publicly blamed Microsoft for Yahoo's lower search revenues.