Was reading Those Tricky Virtual Numbers: Engagement and Investment in Futurelab today and I know why numbers the Yankee Group pulled out for average user engagement (as measured by average time spent per user) has leveled off at just 12 minutes per month.

Like anything else, it all depends on how you calculate metrics and what your underlying assumptions are.  If your going to determine the average number of minutes an "average" avatar spend in Second Life last month you'd need to know the total number of minutes spent that month and just divide by the total number of users that were active that month (entered the virtual world at least once that month).  That makes sense to me than taking all registered users, even though they might not have been active last month.

I looked at the census number for August 2007 but wasn't able to find anything what would closely resemble 12 per month.  I was going to write a bit more here but decided not to.