Kingdon said the population statistic he finds most relevant is the number of hours people spend using the service: User hours grew for the fourth consecutive month in-world in July, to 34.7 million. That figure might be somewhat misleading, because many Second Life businesses deploy automated avatars, or "bots," to make their areas look more populated than they actually are, equivalent to spam on the actual Internet. Kingdon said the company doesn't have a reliable assessment of the fraction of bots that make up its total accounts, but he said the estimates he's seen show that it's "not an uncomfortable number." He declined to be more specific.

But still, Second Life is healthy in one important criterion: Linden Lab is profitable, Kingdon said, although he declined to provide specifics. Au estimates that the annual run rate is $96 million, and $20 million to $30 million of that is profit. That's more than some companies with a much larger cultural impact and user base can claim.

"Silicon Valley is very excited about Facebook, but Facebook is not making any money. It's got a butt-load of users, but how many of them are super-engaged as Second Life users are?" Au said.

Secret Weapon: Passion

The passion of the Second Life community, and the variety of experiences available in-world, are the service's secret weapon, Au said.

Although Second Life is much smaller than other online worlds, the tools that allow users to create, buy, and sell content in-world makes it much more interesting than other, much bigger competition, which are tailored for gaming -- like World of Warcraft -- or teen chat, like Habbo.

"To me, what the half million are doing in Second Life is a hundred times more interesting than what the 95 million are doing elsewhere," he said.

His recent discoveries in Second Life include a Princeton physicist using Second Life to build models of Newtonian mechanics, both as a teaching tool and as a visual reference; and support groups in Second Life for veterans of the Afghanistan and Iraq wars, who meet in sims where there are no weapons allowed.

Au wrote a history of Second Life, the Making of Second Life: Notes From The New World. He considered walking away from Second Life when the book came out this year.

But he continues to blog about the service.

"I can't walk away from all the stories," he said.