A friend recently suggested that every time he's in SL, there is no one around (despite the fact that LL records approx. 444,000 logins per week, which is an average of 63,500 or so per day).
Whenever I log in, there always seems to be between 30K and 55K in-world at the same time (this is up from what... 14K in spring 2006?). Anyway, it seems like a ton of people... so where is everyone??? For those of us who have to on occasion explain things to newcomers and/or naysayers, I did simple math, and it explains things a bit.
Currently, there are 3,952 sims on SL's main grid (http://www.slmaps.com/). This means that if evenly distributed, 50,000 users would result in about 13 users per sim. There are sims/areas that attract many times that number, so the likelihood of finding more than a few green dots in any one location is small. Education builds tend not to attract 'hoi polloi.' Hence, lots of emptiness.
I am not SL's biggest apologist (there are certainly things as an educator that I wish a competitor would come along and fix), but when folks tell me, "SL is boring, " I tell them (with a smile), "So is your course website." Maybe I'm not right 100% of the time, but I've seen a lot of course sites, and looking through them is about as exciting as watching KillDisk zero out a 300GB HDD.
The claim of "no one is there" was put forth by Wired magazine, and a lot of journalists have continued stories along this line because it's easy to replicate the results: Go to Second Life, find no one... and find a bunch of companies pulling out en masse (as reported on APM's MarketPlace on 22JAN08). Their criticism is likely valid for the business world, which wants to put product/service ads & content in front of users. If users are mere vapors, then money spent by companies to use SL (which, all things considered, really doesn't chew up their ad budget too much I suspect) might be better used elsewhere.
Anyway - it's easy to be a critic. As an educator, I'm interested in SL not for the bazillion people I can bring to my plot of land, but what it means for the couple dozen students in my class.