Yammer, a social networking vendor, was acquired by Microsoft yesterday for a whopping $1.2 billion. Yammer will be joining the Microsoft Office team led by President Kurt DelBene. The team will continue to report to Yammer’s CEO, David Sacks until Sacks no longer wishes to be involved with Yammer’s development.

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

Yammer was only launched in 2008, but already has more than five million corporate users. The program allows high-end businesses to join a network of other well-known companies, then enables these companies to tap into the grassroot movement. Most companies expand their beginnings at Yammer into companywide strategic initiatives. Best of all, it’s free for anyone to join.

Yammer plans to continue developing its simple, yet effective, services. The company has begun development of cross-platform applications as well. Microsoft’s current plans are simply to accelerate Yammer’s progress and complement their service with Microsoft services such as SharePoint, Skype, and Microsoft Dynamics.

“When we started Yammer four years ago, we set out to do something big,” Sacks blogged. “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.”