Yammer's software is cloud-based, so it would make the most sense for it to become a component of Office 365, Microsoft's Web-based collaboration and e-mail suite, said Forrester Research analyst Rob Koplowitz.

"I'd assume Yammer would remain a standalone service that could be consumable by Office 365 components," he said. Office 365 includes online versions of Exchange, Lync, SharePoint and Office.

Yammer has 5 million corporate users and its clients include over 85 percent of the Fortune 500. It has raised US$142 million in funding since its founding in 2008 and has about 300 employees.

Yammer a basic free version of its software, and three fee-based tiers.

Poshly, a 10-person Internet startup based in New York City, with staff in California, uses the free version for casual communication, polls, bookmarking and other tasks. Founder and CEO Doreen Bloch finds Yammer very useful for her company but has been disappointed that some features that were previously free no longer are.