Location, Location, Location, that's the real estate mantra that still applies in the virtual world. Somewhere someone is going to create a virtual world where many people (millions?) will spend their time. That's where virtual real estate will have value. Specially if you can get income from it. Virtual real estate is crap now but give it time.
Imagine if Facebook sold a specific area in their website. Let's say it's the upper right inch of every user's screen. What's that real estate worth? Now apply that idea to a virtual world. Ya, you can add real estate forever but only certain areas will have value.
It's the same situation in the real world. You can buy an acre of land in New York city that will cost millions. You can also buy an acre in the middle of the Mojave desert that will cost a tiny fraction of that. They are both real estate. Only New York will be worth millions now and for however long New York keeps it's popularity.