Though a press release issued Monday afternoon, Microsoft has announced their aquisition of the enterprise social network Yammer:

Microsoft Corp. and Yammer Inc. today announced that they have entered into a definitive agreement under which Microsoft will acquire Yammer, a leading provider of enterprise social networks, for $1.2 billion in cash. Yammer will join the Microsoft Office Division, led by division President Kurt DelBene, and the team will continue to report to current CEO David Sacks.

“The acquisition of Yammer underscores our commitment to deliver technology that businesses need and people love,” said Steve Ballmer, CEO, Microsoft. “Yammer adds a best-in-class enterprise social networking service to Microsoft’s growing portfolio of complementary cloud services.”

Launched in 2008, Yammer now has more than 5 million corporate users, including employees at 85 percent of the Fortune 500. The service allows employees to join a secure, private social network for free and then makes it easy for companies to convert a grassroots movement into companywide strategic initiative.

Yammer will continue to develop its standalone service and maintain its commitment to simplicity, innovation and cross-platform experiences. Moving forward, Microsoft plans to accelerate Yammer’s adoption alongside complementary offerings from Microsoft SharePoint, Office 365, Microsoft Dynamics and Skype.

“When we started Yammer four years ago, we set out to do something big,” Sacks said. “We had a vision for how social networking could change the way we work. Joining Microsoft will accelerate that vision and give us access to the technologies, expertise and resources we’ll need to scale and innovate.”

The acquisition is subject to customary closing conditions, including regulatory approval.

Yammer Acquisition: What it Means for Both Companies

In a New York Times article, Evelyn Rusli and Nick Wingsfield go in depth on what this means for both companies and their investors:

Under the terms of the deal, Yammer will be added to Microsoft’s office division and will continue to be led by David Sacks, its chief executive.

Yammer, which calls itself “the enterprise social network,” is a sort of Facebook for business. Its Web-based service allows companies to create private social networks, where employees can privately chat, shares files and collaborate on projects.

“The acquisition of Yammer underscores our commitment to deliver technology that businesses need and people love,” Steven A. Ballmer, Microsoft’s chief executive, said in a statement. “Yammer adds a best-in-class enterprise social networking service to Microsoft’s growing portfolio of complementary cloud services.”

Yammer was co-founded in 2008 by Mr. Sacks, a former PayPal executive who spun the business out of Geni, a genealogy and social networking site.

Over the last four years, the service has grown at a rapid clip, as more companies turn to the Web for secure solutions to enhance interoffice communication and collaboration. Yammer, which offers both free and subscription services, has more than five million users and is used by more than 85 percent of Fortune 500 companies.

It has raised a total of $142 million in venture capital, recently adding $85 million in late February, in a financing round led by Draper Fisher Jurvetson. The company’s roster of investors and board members is stacked with former Facebook executives and members of the so-called PayPal mafia. Founders Fund, the venture firm led by a PayPal co-founder, Peter Thiel, and Social+Capital, a firm partially backed by Facebook, are both major backers. Sean Parker, the former president of Facebook and an executive general partner at Founders Fund, is a board member.

The marriage of Yammer and Microsoft, which also invested in Facebook, makes sense, given the importance of the enterprise consumer to Microsoft and recent moves by Yammer to align itself closer to the software giant. In April, Yammer acquired oneDrum, a British start-up focused on file sharing and enterprise collaboration solutions, for an undisclosed sum.

“When we started Yammer four years ago, we set out to do something big,” Mr. Sacks said in a statement. “We had a vision for how social networking could change the way we work. Joining Microsoft will accelerate that vision and give us access to the technologies, expertise and resources we’ll need to scale and innovate.”

Microsoft’s pursuit of Yammer comes as the enterprise market converges with the consumer Web. Traditionally, a company’s technology purchasing decisions were made top-down, controlled by a handful of information technology experts. That model is rapidly eroding with the rise of Web-based services that are created to be lightweight and user-friendly. Start-ups like Yammer reel in consumers with free versions. Those consumers then become evangelists at their companies, encouraging their employers to adopt the application and pay for a premium subscription.

A Yammer acquisition will allow Microsoft to compete more directly with Salesforce, which offers a similar messaging product called Chatter. Salesforce has also been aggressive in adding social-related applications to its portfolio, recently spending $689 million for BuddyMedia, a social marketing business that works with companies like Ford Motor and L’Oreal. The service helps companies build and manage their advertising campaigns across social networking sites, like Twitter and Facebook.