Google has bought the online ad giant DoubleClick, beating out Microsoft and others in the fray. The cost: an astronomical $3.1 billion in cash, much more than the rumored $2 billion that Microsoft was intending to pay.

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—It will combine DoubleClick’s expertise in ad management technology with Google’s ad platform and publisher monetization services.

—This is the largest acquisition in Google’s history, and comes six months after Google paid $1.65 billion to acquire YouTube.

—DCLK was majority-owned by SF-based private equity firm Hellman & Friedman since 2005, who paid $1.1 billion for its stake. JMI Management is a co-investor in the company.

Here’s how Google describes the rationale of the deal:
—“For users, the combined company will deliver an improved experience on the web, by increasing the relevancy and the quality of the ads they see.
—For online publishers, the combination provides access to new advertisers, which creates a powerful opportunity to monetize their inventory more efficiently.
—For agencies and advertisers, Google and DoubleClick will provide an easy and efficient way to manage both search and display ads in one place. They will be able to optimize their ad spending across different online media using a common set of metrics.”