Let me clarify what "virtual worlds" means. Those who were around in the 90s may remember MUDs and MOOs and etc, which were massively multiplayer games before that term existed. In a sense, social networks today are degenerate forms of this design: we can talk to each other, but we aren't immersed in a sense of place and interaction.
The system I intend to build (Spritely) will allow you and your friends to program your social worlds together. Build it, dream it, make it.
See also :