After I wrote that post yesterday, I was reminded that Robert Scoble had some interesting observations on a related question back in October, under the title New Audience Metric Needed: Engagement. Scoble's piece is interesting enough, but Brian Solis's remarks in the comment thread led me to something even cooler: a recent event in Palo Alto sponsored by Factiva which focused on metrics for social software.
Check out the pointers Brian provides to accounts of the session written by some of the participants. Good stuff.
As a side note, I had to smile at Brian's reference to "return on participation." The whitepaper I and a couple of colleagues wrote on that topic in 2000 is still the top Google hit for the term. To be honest, I was using ROP in a completely different sense that these guys are using it. I was talking about the benefits that users get in return for their efforts.