Honestly, if Microsoft bought Facebook with the intention of OS-level integration, I'd probably start searching deep into my pockets for money to buy OS X or a nice OSS computer setup. The problem with social media integration is that it transforms the "cloud" from just being a very easy-to-use backup solution to pushing for me to expose my personal details in a structured way that the company can take advantage of.
Horrible business advice. Microsoft and Facebook can very well team up to integrate their products as business partners (they simply have to look at Apple and Twitter as an example) without an acquisition and the myriads of possible headaches that will come along with it. HP is still overcoming some of the difficulties that came along with the acquisition of Compaq because of the differences in corporate culture. That transaction is nearing its 10 year anniversary.
I'm already not thrilled about Windows 8. Deep Facebook integration on the OS level isn't making it any more enticing. Facebook doesn't need to be any bigger than it already is, and the consolidation of large companies into an even smaller handful of large companies is something I'm wary of.