IM is interesting because you look at your buddy list and, at a glance, see what your friends are listening to, what they're working on, what they're doing. The problem was that you were bound to the computer keyboard.

I loved couriers. You had this transfer of physical information happening throughout the city and the world. Someone picking up the package, putting it in a bag, going somewhere, taking it out of the bag, giving it to someone else. I thought that was so cool. I wanted to map it, to see that flow on a big screen.

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 first complaint we hear from everyone is: 'Why would I want to join this stupid useless thing and know what my brother's eating for lunch?' But that really misses the point because Twitter is fundamentally recipient-controlled - you choose to listen and you choose to leave. But you also choose what to put down and what to share.

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.

I am someone who tweets about what I have for breakfast, what I have for lunch, what I have for dinner, and for 99.99999 percent of the world, it's useless. It's meaningless. But for my mother, she loves it.

People who are using it to sell things on Craigslist to holding garage sales - campaigns - the Obama campaign and the Romney campaign both used Square to raise funds.

Anything you're interested in the world - whether it be Charlie Rose or JetBlue or a public figure or your local coffee shop - they're on Twitter and broadcasting what is interesting to them.

A number of people in the United States, almost everyone, is using plastic cards to pay for things, but it's extremely difficult to accept these cards. So let's make it's easy and take more and more of the friction out as we can.

When people come to Twitter and they want to express something in the world, the technology fades away. It's them writing a simple message and them knowing that people are going to see it.