I am building a web/mobile social service application for Windows Phone.
As a final goal, I will want my users to have an account at my online service site, but as an application provides a Twitter/Facebook integrated functionality, I don't want to bother client with registering at my site first from the beginning.
But I need to create an account for all user requests to be stored at on the web "under the hood".
What is the best way to uniquely identify an application instance on WP7.5? Is there a unique application installation ID I can use?
You can get unique ID of device by using DeviceExtendedProperties as described in this question: What is the right way to get Unique Identifier of windows phone 7?
UPDATE
With help of UserExtendedProperties you can get unique ID of user as described here. This way you will get ANID (anonymous identifier) based on Live ID. Don't forget to include ID_CAP_IDENTITY_USER in your WMAppManifest.xml.