Microsoft announced Monday plans to acquire Yammer, a maker of enterprise-based social networks, for $1.2 billion. Yammer will become part of Microsoft’s Office division.

Yammer has about 5 million users and excellent penetration in Fortune 500 companies – about 85 percent of them, according to Yammer’s own estimates. The service lets enterprise users create private social networks.

Microsoft says that it plans to “accelerate Yammer’s adoption” through complementary offerings already available from Microsoft, like SharePoint, Office 365, Dynamics and Skype.