Microsoft agreed to buy online social network firm Yammer for $1.2 billion in cash, which will allow the software company to offer a service like Facebook's to corporate customers.

Microsoft and Yammer 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 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. Moving forward, Microsoft plans to accelerate Yammer?s adoption alongside complementary offerings from Microsoft SharePoint, Office 365, Microsoft Dynamics and Skype.

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