It is indeed a lot of people. A quick search says ABK employed 17,000 people. Laying over over 10% of your workforce is... intense, to say the least. Though, how much of that 1,900 is just from ABK is hard to say, so the percentage could be lower.
You're right though; HR, payroll, legal, and social media/PR departments would definitely be among the first on the chopping block, depending on how much MS wants to integrate ABK into their existing departments.