What's a community, anyway?

Earl comments, regarding megacommunities:

You need a special and somewhat sloppy definition of "community" to avoid oxymoronity.

Or in other words, how can you call something with a million or more people in it a "community"?

Certainly the Los Angeles Department of Public Works has no trouble doing so, nor did the European Community, nor do people referring to the scientific community (one hopes that there are at least a million scientists in the world). On the other hand, a horde of 100 million Skype users doesn't exactly conjure up images of neighbors strolling through the park greeting each other with friendly hellos.

If the working definition of "the registered users (or paying customers) of a given service" misses out too much of the "people living and working together" aspect of community, what is it we're trying to capture here? I can think of three aspects:

• Belonging: People will say, for example, "I belong to Facebook" (amusingly, people also appear to be saying "All your X are belong to Facebook", for various X. Ah, the classics.)
• Self-identification: I could define a "community" of, say, people with last names with an even number of letters, but that doesn't make that group a community.
• Interaction: This, I think, is what gives online communities of whatever size their best claim to "community", and why I didn't mention things like banks with millions of online customers as megacommunities.

The whole point of many of the megacommunities I mentioned, and a major point of at least most of them, is that someone belonging to such a community can easily contact another member. Even if they have never met before, they will have something in common by virtue of belonging to the same community.

Whether a given person will actually do this and whether anything will come of it are separate questions, but so are they in a "real" community.