From a product standpoint, we want every touch point to feel magical. It inspires trust.

TweetDeck is a very interesting client, because it presents a view that no other client in the world presents, which is this multicolumn, massive amounts of information in one pane. And people really, really enjoy that.

Success is never accidental.

I'm less interested in how people are following each other and more interested in how they are following topics and tweets themselves. People are following more key words and concepts and more ideas and acting on those rather than individuals or organizations.

The interesting products out on the Internet today are not building new technologies. They're combining technologies. Instagram, for instance: Photos plus geolocation plus filters. Foursquare: restaurant reviews plus check-ins plus geo.

It's a matter of invitations versus context. Twitter is really good at providing context, like, I''m having coffee at Third Rail Coffee.' Foursquare is about invitations to places. In this respect Foursquare has started to replace Yelp for me.

Twitter has been my life's work in many senses. It started with a fascination with cities and how they work, and what's going on in them right now.