MySpace runs Windows?
Jesus, 4,500+ servers running Windows 200x server? What did that cost for the software licensing alone? Let's see, where I work our "Microsoft Volume License" cost for Server 2008 standard is $256.25 per server. So 4,500 licenses at that price is: $1,153,125.00 (at retail: ~$4.6 million).
Even if they got it for half of the volume rate, which I doubt, that's still a sizable chunk of change. I'm no Linux advocate but it seems to me that for considerably less than $1.1+ million dollars they could have hired some capable Linux admins to maintain their systems...
All those servers, all that energy, all that wasted human effort.
Server costs are nothing compared to their bandwidth bill.
The cost of migrating all of those boxes to Linux would be huge now. In the real world you just need to go with the flow and invest the effort in areas that make a difference to your users rather than saving proportionally small amounts changing technologies.
I'm reminded of the saying, "All that's necessary for the forces of evil to win in the world is for enough good men to do nothing".