I ran across a post written a couple years ago by friend Anthony Townsend about Jane Jacobs, Facebook, and urban neighborhoods:

If the physical form of a neighborhood is conducive to community, so is its virtual form. But the other striking thing about the list was that all the neighborhoods were in a state of change—gentrifying or recently gentrified. It’s certainly demographic: a neat and obvious alignment of hipster and blogger. But it also means that the newly emerging character of these places is being forged, at least in part, online. These are incontrovertibly real-world neighborhoods, but their community is as virtual as it is physical. With each year, we get better at navigating between the two.

Facebook and MySpace have begun to show how textured online group interactions can be. It’s easy to think of social networking in terms of Hudson Street, and easy to think of Hudson Street in terms of social networking. Both are at their best when they can successfully balance the public and the private.

Whole thing is worth reading. (So is Richard Florida's comment.)

Of course, there was a time when we talked about urban and online communities as mutually exclusive: remember when virtual communities were going to make cities obsolete? In contrast, today Anthony can assign Jane Jacbos or Christopher Alexander in a class on IT, and nobody is confused.