Tateru, I wonder why people keep looking at things like premium accounts? I've used SL since 2006 and have never required a premium account, leasing or buying my land from others.
One much more meaningful stat, I believe, is the average number of users per hour. While "logged-in in the last 30/60 days" includes alts, and "hours used" is hard to understand by the media (and people not following SL in general), Average Users per Hour is a nice stat, and shows how the use is growing, and how many people, on average, you can expect.
I've compiled those numbers on my blog:
http://andabata.blogspot.com/2008/11/second-life-average-usershour-mdia-de.html