Facebook had 21 percent of impressions -- the number of times an ad is presented for viewing -- in the previous year, ComScore said. Yahoo! Inc. (YHOO) remained No. 2, with 11 percent, the research firm found. Microsoft Corp. (MSFT) was third, accounting for 4.5 percent, followed by Google Inc. at 3.6 percent.Facebook Inc., the world’s largest social-networking service, accounted for 28 percent of U.S. online display-advertising impressions in 2011, expanding its lead in the industry, ComScore Inc. (SCOR) said.
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