What resources are useful to build framework with API like Facebook?

We will develop a web site that will have some free services and we want to make it as a framework that can any build application over it or can use its API in their site.

Could any lead me in how to start it, what's the better architecture and design pattern help in that, is there any resources discuss or explain how to do like开发者_如何学JAVA this ?

We are dot net developers but we can learn any other if it's needed in such a solution.

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The best thing I can suggest would be to use Domain Driven Design and architecture to develop a system that would expose your api as domains that can be programmed.