Microsoft bought Yammer for $1.2 billion

"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."

Source:  Press release from Microsoft, 25th June 2012