But since you mention it it'd make a lot of sense for Microsoft to want to buy Github.
Microsoft has been doing a lot of cool stuff engineering-wise in recent years. They've released so many FOSS projects I've lost track[0]. But they still suffer from a problem of being "uncool" because of their past reputation. Which was deserved, but they've changed a lot[1]. Microsoft needs a new brand for their FOSS work, because the Microsoft brand is badly tainted in the FOSS world.
Github is one of the few companies that could earn them respect in the FOSS world if they were to acquire them (and of course, not screw everything up).
[0] Heck, I've literally lost track of the number of FOSS things they've released this week
[1] Fifteen years ago, could you imagine Microsoft hosting a launch party for Debian? Because that's what they did this last weekend: http://openness.microsoft.com/blog/2015/04/21/microsoft-debi...