A lot of folks around here ask for accurate stats about SL (many claiming that only a few thousand people actually use SL with a ton of alts), so I figured I'd copy and paste a few I found over at Forseti's Blog. (Forseti is also known as Giff, heh)
The number of registered users (Giff: as of this writing 366,332) is growing at 15% a month.
In June 2006 there were 100,000 unique users who logged in
In June 2006, 340,000 items worth US$6.5m were sold and US$1.5m worth of goods were traded directly between users (Giff: need to confirm what is behind this latter figure since a lot of “goods” transferred back and forth do not have an explicit value)
By hours of use, participation is almost exactly 50/50 male/female. Women are more likely to continue using SL than men.
The median age is about 33. The older you are, the more likely you are to continue using SL.
75% of residents are in the US (Giff: I have heard that the international percentage is increasing now that Linden Lab has removed the credit card requirement at registration)
SL gets about 140,000 hours of use per day, around 1/4 of which is spent creating things.
In any 7-day period, about 15% of residents write code (in a rather difficult language similar to C). The monthly total is equivalent to the amount of code in MS Office.
SL is now approaching 3,000 sims. A sim is 16 acres, so the total is closing in on 75 square miles.