For the past four years, Microsoft Research (MSR) has sponsored a symposium on social computing that has brought together academic and industry researchers, developers, writers, and influential commentators in order to open new lines of communication among previously disconnected groups. The 2010 symposium will focus on "city as platform". We will have brief (5-10 minutes) talks by a handful of speakers on each of the topics, followed by related breakout sessions, and lots of time to interact with other attendees.