Grades are not fun. At first, I was uncomfortable with Amy's analysis. I've been part of class-related virtual spaces where I, at least, had a real sense of community. But then I came to the realization that those are the rare instances in undergraduate education where students are really interested in the topic, not just taking it for a good grade in a required course, or it was an instance in graduate education, where students are much more likely to be interested in the discussion as fun. The village square of a MOO is one kind of virtual place that parallels a physical space where we interact with one another.
Communities involve committment.
It takes time to become and remain a full, contributing member to a community. Other social interaction spaces may make different trade-offs. Cocktail parties and meetings are two kinds of social spaces where people interact that are not in themselves communities. Cocktail parties and meetings don't require as much committment as a community. One can attend a cocktail party or meeting only once and never re-meet the same people.