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.
There has been a series of recent patent deals between technology companies as they try to defend themselves in lawsuits.
If a company successfully sues another it can demand a sales ban of its competitor's products, or force the loser to pay expensive licence fees.
Since the start of the year, Intel, Google and Facebook are among those to have bought significant numbers of patents from other technology companies.
Facebook bought a number of patents from IBM last month.
Also on Monday, Facebook reported results for the first three months of the year.
The company, which is expected to list on the stock market in the coming months, reported a drop in its net income between January and March to $205m from $233m in the same period last year.
Revenues for the quarter came in at $1.06bn, down 6% from the final three months of 2011.
Facebook said that its advertising business usually slows down in the first quarter but that the growth of the business in previous years had masked that trend.
Facebook also revealed that it has agreed to pay $200m to Instagram if its recent $1bn deal to buy the photo-sharing firm were to fall through.

Attendances have fallen in recent years since the Premiership side moved from Edgeley Park in Stockport to AJ Bell Stadium in Barton in 2012.
Sale have moved home games back to Friday nights for the upcoming season.
"I think there is some rebuilding to do because I think people in the early days of moving have had a pretty poor experience," Cotton told BBC Sport.
"That could be queuing up on a motorway, couldn't get away from the stadium, couldn't buy a drink at the ground. Listening to the things we're going to do to change that, if people come down from now they will find a very different experience.
"The infrastructure is still well away from where it needs to be but there are plans to develop that."
Cotton, 69, a former England and British and Irish Lions prop returned to Sale - a club he served as both a player and director - after Simon Orange's takeover of the Premiership side in June.
He has promised to be active in his role, despite his commitments with his clothing business.
"What I really want to see is a very strong north of England club that can compete at the highest level," he added.
"That is my motivation for giving up the precious thing that I have got, and that is my time.
"I think what was lacking before was that the board weren't very often represented at games home and away, and I think that is very wrong - board members will travel to away games and home games."
One of the standout announcements from Sale's new 10-man board was that director of rugby Steve Diamond will have a seat on it.
However, Cotton insists there will be no conflict of interest with a member of the coaching staff also working as a director.
"It will put no strain whatsoever on the relationship, we're here to support him," Cotton continued.
"We'll be working very closely with Diamond to make sure we can achieve the ambition which we all hold - if we went down we'd still be with them."
Diamond also confirmed they are close to signing a new full-back and are still looking for another player to add to their backline, with former Bath and London Welsh winger Nick Scott on trial with the club.
Facebook is paying Microsoft $550m (Â£341m) for some of the patents it recently bought from AOL.
[NEXT_CONCEPT]
New Sale Sharks chairman Fran Cotton has conceded they must work to restore their relationship with some fans.