Those are applications, not platforms. Platforms are intended to make it easy for independant developers to build value-added applications. For example: MySQL, MS Office, Common Lisp, Amazon EC2, etc.
Yes, and I was saying that xobni should build out a platform for developers to create VALUE ADDING applications to gmail, yahoo & hotmail, like To-do lists, contact lists, crm's, etc...
It's generally unwise to a build a platform on top of something controlled by third party. Any little change that third party makes is likely to break your platform.