One way to think about cities is as places where you are always surrounded by strangers. Walk down a given block and most people you've never met before. The pedestrian crossing the street in front of your car, your bus driver, and the person you find on craigslist for your empty room are likely to be people you've never met before.
The question, then, is not "How do we build for a tight knit community?"
The question is "How do we build for strangers?"