Total Residents: 24,163,789 - registered SL accounts but this data LL don't release anymore and if they do somewhere they won't say anything about it.
Data stopped for residents who logged in the past 60 days during late 2010.
http://www.gridsurvey.com/economy.php
Check out Q1 SL metrics for 2011. LL releases metrics every quarter of the year.
http://community.secondlife.com/t5/Featured-News/Q1-2011-Linden-Dollar-Economy-Metrics-Up-Users-and-Usage/ba-p/856693