Azure as a platform for Facebook
I think they are both good platforms for Facebook apps as they support building web apps that can scale if they become popular. I will not compare further as I am clearly biased. (I am working on Windows Azure.)
However, in case you're curious, I wrote a blog post about my own Facebook app running on Windows Azure: http://blog.smarx.com/posts/facebook-apps-on-windows-azure . It's written in .NET but we support PHP as well.
I am currently working on a facebook app hosted on Azure.
I don't think there is any advantage of Facebook over Google App Engine, but azure is a fantastic platform for quickly deploying test applications.
Most of the answer will depend on the code you choose. Will azure support most languages, some are better than others in terms of tools at the moment.
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