AOL (NYSE: TWX) has lost another top exec at its advertising unit Platform-A, as Curt Viebranz has stepped down from the top job as president of the unit. In his place, AOL is promoting Lynda Clarizio to president of Platform-A. Clarizio has served as president of Advertising.com, one of the several parts that make up Platform-A. She will report to Ron Grant, AOL’s president and COO. Viebranz, the former CEO of recent AOL acquisition Tacoda, became Platform-A’s first president when the organization was announced last fall. No word yet as to why he left or where he’s headed.

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Before joining Advertising.com in May 2006, Clarizio, was an EVP at AOL in charge of strategy, finance and business development of AOL’s Audience business. She has held several other positions throughout her nine years at AOL, including SVP for corporate strategic and financial planning and SVP for acquisitions and strategic investments. In the latter role, she led AOL’s acquisition of Advertising.com. Clarizio came to AOL from the Washington D.C. law firm of Arnold & Porter, where she was a partner for seven years.