Microsoft's rumored acquisition of enterprise-focused social networking service Yammer is seemingly all but confirmed, as The Wall Street Journal has corroborated earlier reports of a $1 billion buyout. But according to our sources outside of the company, Microsoft will not be announcing the deal at its mystery event on Monday, and the details of the acquisition may not emerge for some time. The acquisition is believed to be part of a larger strategy to boost Microsoft's social networking offering, potentially leveraging Yammer's strengths as a private, internal communications tool for businesses. We have reached out to Microsoft, but the company declined to comment.