No timetable was given for the closing of the deal, which is subject to regulatory and other approvals. News of a possible deal between Microsoft and Yammer emerged on June 14, when, citing anonymous sources, Bloomberg that Microsoft was in advanced talks to acquire Yammer for about US$1 billion.

Yammer's software is used by more than 5 million corporate users and in more than 85 percent of the Fortune 500 companies. It a basic free version of its software and three fee-based tiers. Yammer, which has raised $142 million in funding, has about 300 employees.

About 200,000 businesses use Yammer in more than 150 countries, DelBene said. One such customer is Deloitte, which rolled out Yammer to 190,000 employees, he said.

Yammer adds about 250,000 corporate end users every month, according to Sacks. Its other customers include Tyco, Ford and Nationwide Insurance.

The deal triggered reactions from several Yammer competitors who emailed statements to the press.