You can almost imagine using transports to do this right now (register with the linkedin transport and it injects the linkedin buddy list into your roster and now we can grant access to those entities). But I think there might be a reasonable case for having first class support for this kind of stuff.
This is a really interesting idea actually.... and is very much in line with the 'federation' part of this discussion. You may have users only on facebook, or on other services all mangled together in your roster by allowing there to be relationships between 'rosters' on different networks within your roster. A FB ID can easily be an alias for a JID in your roster. I'm not sure how this all really plays out, but it is an interesting line of thought. Either way, people will be looking for solutions to this problem and it can't hurt to have ideas on the matter.
adam
