Facebook is paying Microsoft $550m (£341m) for some of the patents it recently bought from AOL.
Microsoft paid more than $1bn for most of AOL's patents, beating rivals reported to have included Facebook.
Facebook was sued by Yahoo for patent infringement earlier this year.
The social networking site, which is preparing for a stock market listing, also reported a drop in its first quarter profits to $205m from $233m a year earlier.
A Facebook lawyer described the deal as: "Another significant step in our ongoing process of building an intellectual property portfolio to protect Facebook's interests."
Microsoft bought 925 patents and patent applications from AOL. It is now selling 650 of those patents to Facebook as well as licences to the other 275.
"Today's agreement with Facebook enables us to recoup over half of our costs while achieving our goals from the AOL auction," said Microsoft's general counsel Brad Smith.
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