The virtual worlds consultancy kzero.co.uk reports that membership of virtual worlds grew by 39% in the second quarter of 2009 to an estimated 579 million. Some of these memberships are likely to be unused or inactive but this is still a massive rise. Much of the growth comes from children, particularly in the 10-15 age group.
Habbo has in the region of 135 million members, and allows users to create and connect, it is incredibly popular with teens. Second Life allows users to create their own content, unlike most of the newer virtual worlds. Whilst OpenSimulator lets you create a virtual world on the hard drive of your own computer, linking to other compatible ones, such as Second Life.