@maiyannah I think that "namespaced by instance" is OK.
E.g. user ID: acct:andstatus@loadaverage.org
message ID: msg:208167@loadaverage.org
The main point is that in order to request the same user or a message from different GNU Social instances, one should use the same Global ID.
I.e. the same URL path will be used to request same message from any instance, e.g.:
/statuses/show/msg:208167@loadaverage.org
- for ANY instance, not for loadaverage.org host only.
Plus such URIs allow to find out a source of the URI very easy...
@takeshitakenji @archaeme