I’ve found the article „Second Life: Hype vs. Anti-Hype vs. Anti-Anti-Hype“ from the midth of December 2006. A very well thought article from Wagner James Au about the recent developments, company side and growth in SL. Definitely worth a read, so not a short one, so take your time!
[Update]: Seems the user base grows about 23% per month according to this artice based
on numbers bei Philip Linden.