One of the problems of the corporate world today is lack of focus. We have food stores selling motor oil, gas stations selling milk and pharmacies selling everything. I can't help but feel that the net result is a descent into mediocrity.
Microsoft as mentioned, has Hotmail and MSN, both of which compliment a software manufacturer by providing some basic tools to stay informed and browse the web. To add Twitter to the mix is akin to selling popcorn at an auto parts store - You could, but why would you want to?
Now, I'll admit that I am biased against social networking in general, which I consider to be the equivalent of organized spam, where 99.9% of the content is nothing short of rambling inanities, in my opinion. I think that Microsoft would do well to concentrate on what it does best and leave the 'fluff' to others.