New World Notes has a recent post that points out some interesting data. Tyche Shepard estimates that 80% of Linden Lab's revenue comes from 5,500 SL residents, and 73% comes from just 500 people.
It's rather reminiscent of the distribution of money the US government takes from us via income tax; a few at the top end pay the majority of that money.
And in a way, it is a tax; in the virtual world, it is essentially property tax, the money that landowners pay for the privilege of keeping the virtual land they buy. (Can anyone really be said to own property if they have to continually pay the government to keep it from being seized? I digress...)
On the other hand, it's not like a tax in one important way: unlike the government, you have the choice to not deal with Linden Lab. How many, and what kind of, missteps would it take to drive away the 500?