Web applications can be considered as virtual spaces. There’s no need to be Second Life to be a virtual space. And I will drop the “virtual” adjective too, because I think there’s nothing virtual about the relations and the experiences happening on those spaces.

I think I will call them “experiential spaces” (or maybe “possibility spaces” *wink*). The name is not important, though. Raph Koster has this great article about empowering social interactions inside a whatever space. And I think they should be kept in great consideration when designing a social web application in a playful way. Check it out.