2. DoubleClick
2. DoubleClick
Price: $3.1 billion
Announced: April 2007
FTC Approval: December 2007 (March 2008 in Europe)

Of the big three, DoubleClick will be the most obscure name for the layman reader. But it shouldn't be, given the huge role DoubleClick's display ad business plays for Google. Regulators knew that, which led to protracted investigation, especially in Europe. Once given the green light, Google made DoubleClick its core business for display ads, what analysts had considered a weakness for the company. Reports from the time noted the steep price tag but applauded Google for taking steps to catch up with Yahoo! and Microsoft in such ads. DoubleClick was the first, and biggest, of Google's major advertising capability buys -- what CitiGroup called Google's "very expensive but very significant strategic thrust."

Last updated August 16 2011: 12:14 PM ET
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The $12.5 billion acquisition is guaranteed to send shockwaves through the mobile sector. Truth is, both companies need each other badly.

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