It has been a big week for Microsoft Office, with the release of the preview of its next major version. Now comes word that Microsoft has completed its $1.2 billion acquisition of Yammer, the business-oriented social network, which is joining the Microsoft Office Division.

“We couldn’t be more pleased to welcome the Yammer team to the Microsoft Office Division,” says Jared Spataro, senior director in the Office Division, in a post announcing the news. “We’re excited about the road ahead. We look forward to accelerating the adoption of Yammer’s standalone service, and enhancing the social capabilities in Microsoft’s communication and collaboration offerings.”

Yammer offers a Facebook-style interface used for communication and collaboration inside companies. Its customers include Vodacom, Ford Motor Co., LG Electronics and many others.

Explaining the deal, Microsoft CEO Ballmer said previously that he sees value not just in Yammer’s products, technology, employees and customers, but in its “viral adoption model” — in which individuals and groups can start using the product initially on their own, in many cases leading their corporate IT departments to sign up for premium management and administration tools.

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