A friend of mine that works in MS uses it for Skype-related development. MS have deployed a number of Linux-based Skype supernodes to improve connectivity.
they've even got commits in the Linux kernel!
They have commits in the kernel just because they want to run Linux on Hyper-V and they need drivers to support it.
They we're later criticized for not maintaining the code and etc. But after that I think that MS improved the code and continued commiting updates.