If you're instant messaging someone on the company's IM platform on the company's time why the fuck would you have any expectation of any sort of privacy?
I know my company can see everything I can do when I'm logged on to their computer. This is part of the agreement I signed with them. It's also the reason why I don't do stupid shit on my company's network like look for another job or send out resumes from my company email address.
Oh wait, the outrage is because it's Microsoft. Got it.