Here is an interesting chart comparing "Casual Immersive Worlds" by target audience, main premise, immersion level, graphics & point of view, number of users online when visited by surveyor, June 2007 total unique visitors, and revenue sources.
Some libraries are trying to set up shop in Second Life, but I think once you do that, you are really reaching outside your local target audience.
What can we learn from these sites? I don't think we can afford to create a immersive library world for our teens. How can we create features in our website that provide the feeling of community these sites offer?