The Azure Active Directory Technical Documentation Library page reads "As a highly-available and highly-scalable service of Windows Azure, Windows Azure AD can be used to manage identities of both small and large organizations."
Would you quantify "highly-available" and "highly-scalable"? If you need to know what I'm trying to do before answering, quantify "highly-available" and "highly-scalable" in the granularity required to separate the different scenarios which may have more or less scalability and availability.
Source: http://msdn.microsoft.com/library/azure/jj673460.aspx