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Yahoo's currently second with 11 percent of the market, comScore says, while Microsoft, Google, and AOL trail with less than 5 percent apiece. ComScore has placed Facebook No. 1 in the display ad impressions category since 2009.
As Facebook - which may file for a public offering as early as Wednesday - has grown to more than 800 million users worldwide, brands have gotten on board with vigor. The Menlo Park, CA-based digital giant has successfully courted Madison Avenue during the last two years in particular.