How should networker metrics be extended?

I think your question is very interesting -It is certainly a challenge worth understanding. I have assumed the question is aimed at a LinkedIn type of social/business community. I have assumed that you are asking about how to measure

1. NEW member growth
2. Extension of networks amongst those who have already "found" LinkedIn.

Here's my brain dump:

Part 1. Macro growth : If you are looking for where macro growth takes place, I think you would have to have a fairly sophisicated model. I would include things community participant promotion, on-line/off line advertising, partnering, geography, job function, etc.

I don't yet see how you can "verify"/attribute new growth easily. Given the proliferation of electronic media (using multiple computers), and a secruity focus on "removing all internet traces", it may be impossible. I recall that a "new buy" decision may involve seeing the same "ad" a dozen times over a long period of time - rarely is it the "first" advertiser who gets the sale - unless they keep in constant contact.

Pragamatic solution - ask new members where they remember seeing the advert first... and what made them decide to join now (assuming this is not the first time)...

Specifically LinkedIn What I do: I promote in two ways - having my profile link in my email signature & business cards AND recommending the service to candidates (whether or not I can help them find a good career opportunity). I'd say roughly 5-10% are already on LinkedIn (but did not include their link in their cv); 15-25% join and ask me to link. I don't mind if they don't want to link to me afterwards - it is their decision.

I think the question for any network community is to figure out where there are pockets of valuable members who are not yet being reached - I suspect that as the world becomes "flatter" the divide with be between those that choose to participate on-line those who choose not to do so.
I certainly get lots of invitations to join other on-line communities, so it's probably worth asking the question where you think they might be more successful.

Part 2:(I think that) The success of an on-line community rests with users getting a valuable experience - eg identifying & participating. Extending/strengthening the on-line networks is paramount. So the recommendations, forwarding profiles, introductions etc, would need to be measured. In terms of influence, I think the Answers/Experts are useful for those who have the time to participate and a huge success I suspect.

However, I think many people measure the value of their network on their reach (as answered earlier)