The service is known intimately by only 20 or so beta testers at present – the company officially launched this month with a 99-character tweet sent out by Mr Dorsey: “Announcing our new company, called @Square, which I'm thrilled to be a part of: http://squareup.com,” he wrote.

One tester is the Sightglass coffee bar, another Dorsey investment, which is currently a kiosk but will become SoMa's largest café at around 7,000 sq ft when finished.

“My mom owned a coffee store when I was 12 or 13. I love common spaces, and coffee houses have a rich history of affecting the democratic process,” says Mr Dorsey.

That interest and Square also dovetail with his investment in Foursquare, a location-based social networking service that makes a game of discovering new locations. Users check into places of interest and earn rewards for doing so.

He also has a seat on the board of Ustream, which allows users to broadcast video on the internet from a phone or webcam.

Mr Dorsey ventured out of his SoMa comfort zone recently to visit Baghdad and Mexico City with the USState Department. It was part of his evangelising work with Twitter, where he is still chairman and in-volved in major strategic decisions.

In Mexico, he suggested a Twitter solution that he says the government is now working on to combat drug cartels. Users would be able to record incidents of violence and send reports to the police and a public database.

He says: “It's about having access to a worldwide broadcasting system like Twitter with a $20 phone and how that can expose things and lead to more positive behaviour and change.”

With his new project, Mr Dorsey is finding that the phone can empower users, with the help of a small square dongle, to change the balance of power in the financial system.