You should point out that XenServer is almost exclusively for quite small companies, not large ones. XenServer goes after the SMB market primarily. It is Xen without XenServer that tends to go after the big enterprise customers. Both play in both markets, but XS really is focused on SMB and Xen sans XS on the enormous cloud market.
But as such, those big clouds are used by nearly every up to date SMB. So almost everyone is running something on Xen even if they do not realize it. Xen is just about everywhere.