Can a Windows Live ID be used to access a User's Facebook, Twitter, etc information? Is there a Windows API to do this?
Or... do apps need to be created for each of the different social networking sites and each of their different REST API's need to be used to access their content?
You probably need to talk to the authors of those sites for this. They definitely could use the LiveID to log if they wanted. You can find out more about the Live API at the
Live Connect Developer Center and on the
Live Connect forums.
Apps for the social networking sites can also share with more general apps via the Contacts, Sharing, Picker, etc. contracts. If you have questions about those please post in the
appropriate forum for the language that you are using.