I just want a little clarification concerning hosted CMS like shopify.com, solidshops.com (I learnt Shopify runs on Ruby on Rails) so let me be specific about hosted CMS based on Drupal: www.buzzr.com, www.drupalgardens.com and www.pagebuild.net etc..
What I want to know is
Do they use the Multi-Site feature in Drupal to automatically creates all those 1000s of sites they host when a user sign up?
Do they create those 1000s of sites as sub-sites (otherwise known as subdomains)?
Do they use a different way other than the Multi-Site in Drupal?
I don't know how Buzzr and Drupal Gardens work, but there is some info about Pagebuild in this Pagebuild Case Study. Hope that helps.