In the process of pitching Second Life to various clients, I decided to answer just this question. Of course, I don't have a crystal ball, so instead I resorted to using Excel (I think there's a marketing slogan in there somewhere: "The next best thing to a crystal ball: Microsoft Excel".
All jokes aside, I think the results of simply
applying a compounded monthly growth rate are surprising (we all know
how hard it is to calculate compound interest). What this shows is
that if the current rate of 22% monthly growth continues, we're looking
at 3.6 million residents by July 1 2007. If things slow down and SL
grows at just 10% monthly, there will be 936K residents.
If we decide to split the difference, we're still looking at a couple million users a year from now. Pretty mindblowing.