Strategy Analytics has a pay-for report (it's the day for 'em) looking at projected virtual world takeup over the next ten years.
This study, titled "Market Forecasts for Virtual World Experiences," predicts that 22 percent of global broadband users will register for one or more virtual worlds over the next 10 years. This will expand the virtual world market to one billion registrants, with roughly an eight billion dollar services opportunity.
... says GameDaily, who have presumably bought a copy. Of course, registering for and actually using are two entirely different things, but with the number of virtual worlds in development, this chart is probably not a massive surprise...