The company has been expanding so rapidly that it had been considered a prime candidate to pursue an IPO by next year.
The stock of Yammer rival Jive Software has gained 65 percent since it went public six months ago, though it fell 74 cents Monday to close at $19.75.
The IPO market has gone into a deep freeze since Facebook flopped in its closely watching stock market debut last month. Instead of soaring as had been widely anticipated, Facebook shares plummeted during the first few weeks of trading. Although the stock has rallied recently, it remains 16 percent below the IPO price of $38, which had minted Facebook Inc. with a market value of $104 billion. Facebook shares dropped 99 cents, or 3 percent, Monday to close at $32.06.
Sacks said the frosty conditions in the IPO market didn't influence Yammer's decision to sell to Microsoft. The negotiations between the two companies began before Facebook's IPO, according to Saks and Ballmer.
"Our thinking was based on the fit with Microsoft and the fact that we think Microsoft is a great partner for us in expanding the service and taking it to the next level," Sacks said.
The sale will provide a hefty return for Yammer's early backers. The startup has raised about $142 million in venture capital.
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