UPDATE: Bloomberg is now reporting that Microsoft is in talks to buy Yammer for more than $1 billion, with a deal coming as soon as Friday.

EARLIER: We just heard from a source inside Yammer that the office has been abuzz since Monday with talk of Microsoft buying the social-enterprise startup.

And the talk is spreading outside Yammer's office hard by the Caltrain station in San Francisco. Sarah Taylor, an administrative manager at Ignition Talent Group, tweeted about overhearing people talking about Microsoft acquiring Yammer at the Creamery, a café across the street from Yammer's office.

Yammer's Web-based software lets employees work together on projects and share information in an environment that looks more like Facebook than Microsoft Office.

Since 2010, it's offered some integration with Microsoft SharePoint, a cooperation which it expanded in April, along with support for other third-party apps.

Buying Yammer would also thrust Microsoft into the hot market for social-enterprise tools—putting it in competition with Salesforce.com, Yammer's archnemesis. Salesforce has a Yammer knockoff called Chatter.

Yammer raised $85 million in February at a reported valuation of $500 million.

An acquisition deal would have to be well north of that—maybe even double.

A Microsoft spokesman said the company wouldn't comment on rumors or speculation. Yammer hasn't responded.

Now see how Yammer employees spend their evenings >