To store those cute catpics you take with your iPhone, Apple is using Microsoft's Azure cloudservices which run on Dell hardware and have a Microsoft core and might run Windows Server (but probably Linux).
I was happy to kick out the last Dell and Windows out of our office years ago. Oh the irony.
Disclaimer: above info might not be technical correct. I have no experience whatsoever with Azure.
I bet you 3 cute cat pics Microsoft's servers are running Linux.
How up to date is that info, by the way? I would have thought with all their recent investment in their own DCs Apple weren't using somebody else's anymore.