Microsoft is in talks to acquire business-oriented social network Yammer for an estimated $1 billion (£644 million), several sources are reporting.

The relatively high price for a network that remains unproven despite its claims of high take-up is said to reflect a desire by Redmond to add business social media to its stable in order to battle Facebook, Oracle and All are investing in similar services.

This fits with conventional wisdom about the need for larger enterprises to build internal social networks that don’t simply repurpose systems designed for the public. Yammer also integrates with enterprise systems such as SAP.

“Five to ten years from now every company will have an internal social network and maybe an external one as well,” Yammer CEO, David Yammer, reportedly said.

Long rumoured to be looking at an IPO, Yammer raised $85 million in venture money as recently as February, putting off the need to go public to find more capital.

The company recently opened its first developer centre outside the US, in London’s Tech City. Forrester recently estimated the market for business social media at $4 billion.