Earlier reports by Bloomberg suggested that the deal would be completed by the end of Friday (Jun 15).
The deal could bolster competition with Salesforce.com which obtained social tools after purchasing Buddy Media for $745m earlier this month.
Buying Yammer would also give Microsoft a leg up against Oracle which bought Virtue and Collective Intellect which analyze data on social networking sites.
A WSJ source said that "it is unclear when the acquisition will be completed and announced."
Yammer was started in 2008 by David Sacks, the former COO at PayPal. The company is currently used by over 200,000 companies and is 100% cloud based.
The company raised $85m in its fifth round of funding back in March bringing its total to $142m in venture funding so far.
The company claims it has more than 80% of Fortune 500 companies as clients.
Unlike Twitter and Facebook, Yammer offers an internal social network for companies where they own and control all their data on yammer.
In a 2011 interview, David Sacks, CEO of Yammer told CBR how fast the company was growing.
"Salesforce claims four million users, we’ve got four million and we’re a younger company," said Sacks.
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