Drawing on a credit card or a linked bank account, users send money to another person in much the same way they would a text message. The app lets users include a note about what the payment was for, similar to using a paper check. It also invites users to share their notes on their Venmo activity feed and on the social networks Facebook, Twitter and Foursquare.

Venmo claims that, prior to moving out of a two-year private beta Tuesday, it was processing roughly US$10 million a month.

Payments don't jump out as a real-world activity that users are clamoring to share on social networks, but, according to Venmo co-founder Andrew Kortina, most payments have been shared.

"It's not obvious at all, which is maybe why no one has done it," Kortina said of sharing payment information. He and co-founder Iqram Magdon-Ismail, who were freshman-year roommates at the University of Pennsylvania and have been friends since, developed the app for themselves and then wanted to notes with their payments to one another. But they weren't initially sure other people would use the app the same way.

"If you go out with your friends to a bar, a movie, a restaurant or a trip, all these things cost money," Kortina said. "And all these fun things are things you're going to tell your other friends about who weren't there. They're already social things that you're paying for, so it sort of does make sense that you would want to share them with your friends."