1.) I have not tried this out much myself but there is a demo SDK-addon available in the platform distribution. Look in the vendor/samples folder of the Android Open Source Project. There is not much documentation on it. What is needed is that you create product make files for your SDK-addon and build your platform with this product description. It will generate the file system images needed to include with the SDK, documentation etc.
2.) I am not sure if SDK-addon is suited for connecting social services like you describe. Using a regular Java library and linking it directly with your application sounds like a better way unless your are building for a specific device. As I understand it the SDK-addon and framework extension mechanics are primarily for people who create their own devices and need to add device specific API:s to the SDK. One example would be the case where you are using Android to build a navigation system in a boat and want to include API:s where you can get information about speed, engine status, measured depth etc from some other hardware in the system. In that case you would build your system including the extra services and then generate an SDK addon for developers who would like to build applications specifically targeting your nautical use case.
3.) Yes, I believe you can. Remember however that SDK addons require system images that correspond to a device with the added capabilities and applications written using the SDK-addon will only work on devices that include this functionality.
