Via Mashable two days ago (emphasis mine):
Microsoft is investing $15 million in Foursquare as part of a licensing deal that will incorporate the the location-based service's data into Microsoft's web and mobile platforms.
The four-year deal, announced Tuesday, makes Microsoft the largest licensee of Foursquare data.
Me, just 5 days before that regarding how Foursquare continues to grow and evolve (again emphasis mine):
I predict they will keep adding features until they get purchased by a one of the major tech companies like Google, Apple or Microsoft. The data alone is worth it.
Did they outright buy Foursquare? No. But I was right on the money. This all makes sense though. Foursquare must have some of the best data around when it comes to data about what we do every day. And this kind of data can easily be used to make money.