A recent paper by Matteo Varvello et. al. answers a lot of the questions I've found myself asking about Second Life and its community. The paper Is There Life In Second Life? and if you're interested in Second Life's popularity, community, and performance, it is a must-read!
A quick inspection of recent web articles about Second Life shows a common sentiment that perhaps popularity its is waning - or was never as great as total user account statistics suggested. When user comments are enabled on these articles, you also see that whatever their numbers, Second Life aficionados are extremely vocal when defending the popularity and functionality of their world.
It'll be interested to see how the Second Life community reacts to this paper.
It presents a lot of salient statistics about how many users are online at a time, and what those users are up to - at a macroscopic level.
So check it out! Is There Life In Second Life? is extremely accessible, so don't worry about needing a PhD to make heads or tails of it :)