Besides being some of the hottest start-ups around, Rovio, Uber, Fab.com, Airbnb share a common thread: their payments are powered by Chicago-based Braintree, a payment provider that has quietly become a force in the online commerce world. The company, which raised $34 million from Accel Partners last year, is now processing $4 billion annually and is on pace to break $1 billion for mobile in the next month.



