The report said that the software giant will pay between $300m and $500m for the stake.
If the figures are accurate it would value Facebook at up to $10bn and make its founder Mark Zuckerberg a billionaire.
"It would probably be pretty good for Microsoft since it has not had the best success in creating really hip, young-people-grabbing stuff on the web," Kim Caughey, a senior analyst at Fox Pitt Capital, told The Guardian.
The figure is a step up from analyst predictions last month that Facebook was worth around $6bn. The company had a turnover of $150m last year, with profits of $30m.
However, Facebook has several hurdles to overcome, not least the decision of a court as to who actually owns it.
A legal battle is currently underway between Zuckerberg and former college friends about who had the original idea for the site.
The company is also being investigated by the New York Attorney General over allegations that it does not protect its users against sexual predators.
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