Yes there are some very clever and creative people out there who can come up with these ideas. These virtual worlds certainly reach a huge audience. I would say that these sites could become very addictive very quickly as you become more and more involved the the "virtual" world with the characters you create. I also think it could be dangerous not just for young people but adults as well who want to "escape" reality and would rather live in the virtual world.
Being a visual person myself I can see its uses educationally, the 3D aspect of the software could make learning more appealing rather than just text or video. Having said that I am not interested and dont have the time to "play" in the virtual world.