Ready Now

Any service, big or small (consider Craigslist.org, for example) could do any of these today:

have us sign in using the ONE-OF-US.NET phone app as ourselves and using (or not) their own delegate keys
implement its own phone app to replace the ONE-OF-US.NET phone app (your keys are yours and available for export)
read, verify, and leverage the content you've signed and published elsewhere
start publishing content signed by our own delegate key for that service and make it available (or not) to us all

This (and only this) is what the Nerdster and the ONE-OF-US.NET phone app use. The phone app stores your identity private key on your phone, and Nerdster can remember your private delegate key, but otherwise, both apps read and verify your own signed, published content as they load up just like other services would to leverage that data.

QR Sign-in

There is no special access between one-of-us.net and nerdster.org. The ONE-OF-US.NET phone app reads the Sign-in Parameters in the QR code, checks if a delegate key for ("domain") is present, and then performs an HTTP POST over the Internet to the address ("url") it was instructed to, optionally end-to-end encrypted using the disposable encryption public key provided ("encryptionPk").

When signing in to a service on the same device as the ONE-OF-US.NET phone app, these 2 are offered: https://keymeid (a "URL scheme") is registered by the ONE-OF-US.NET phone app. It's heterogeneous and works reliably on Android; on iPhones, not so much.
https://one-of-us.net is obviously not heterogeneous. This tech goes by "Universal Links (iOS) & App Links (Android)", and it does work reliably on iPhones.
The data communcited is the same as what's in the Sign-in Parameters.

Our signed content is published

For example: My signed content is here: identity, Nerdster content.
Both services publish all signed content at https://export.<service>/?spec=<key token>.

Identity statements so any service can compute who's who.
Nerdster follow statements so any service can compute who Nerdster follows who and for what (if they care ;), and all the rest of it too: ratings, comments, equate/relate statements, censor statements, etc...

Services are free to choose

Other services don't need to be as naked and transparent as the Nerdster, don't have to use delegate keys (just identifying a Point of View (PoV) might be enough), and they don't have to expose folks' signed content, although this enables other services to audit, and that can enhance credibility, which is nice.