@emacsen I mentioned WebFingerId not as a part of WebFinger protocol, but only as an artificially created identifier, helping to figure out the same actors via old-style (Twitter-like...) client APIs (which don't provide globally unique identifiers of actors (users...)).
As we're currently are figuring out, how to use # C2S protocol in the real world :-) I see that WebfingerId isn't really needed in ActivityPub API: Actor's profile has enough information to know both username and hostname of an actor (what constitutes WebFingerId).
I think that you will be interested to read our discussion on this here: https://github.com/andstatus/andstatus/issues/499

@cwebber @thefaico @eliotberriot