I did a programmatic hosting survey of 300+ startups funded by Y Combinator just a few months ago. Of those, unless they're one of the 7-8 whose main site was being served entirely from a CDN, none are hosted on Microsoft Azure. Amazon EC2, Rackspace, Softlayer and Linode together hosted about 3/4 of the companies.
What were some of the other hosting options for the other 1/4?