Interesting web platforms...

Web Platforms form the "operating system" of large scale web applications. What kind of web application are we talking about? They're more than just a few web pages and a static store of page content. Below is a list of some interesting examples to compare and contrast, and for each one I've highlighted one of the "key values" that they provide.

• LinkedIn: The key feature is "the graph" of people and their relationships
  
• Wikipedia: It's really just collaborative editing of simple pages, but they have to be available to the whole world!
  
• salesforce.com: A platform for building applications, users, access permissions, data stores, etc.
  
• facebook: The extensibility of the "widgets" and how they use backend information sources
  
• Amazon's EC2 & S3: Virutalization is the core concept.
  
• Slashdot: Group editing and rating
  
• Google's Search: map reduce farm creating the indices AND the high availability architecture