In each corner of the startup world there's a Venmo for every Paypal -- a little engine that could, and wants to, badly.

FORTUNE -- Hours before I met Andrew Kortina, I had already given him my credit card number. I don't do this with just any stranger. But Kortina and his cofounder Iqram Magdon-Ismail have built a service called Venmo, which promises to make mobile payments so easy, people will actually start to make mobile payments. And after a lunch bill that my friend refused to let me pay for in downtown Philadelphia, I was determined to make my friend take my money. So I downloaded the Venmo app, registered my account, specified the amount I wanted to pay and to whom, and typed in those 19 numbers (including the three on the back of the card) that control my finances.

All of this will not sound terribly novel. That's because I -- and, no doubt, many of you -- did something similar years ago with Paypal. And PayPal, as is expected of a huge company with thousands of employees, also has a mobile app. One that I could have used just as well as Venmo's to pay my friend back. This is Venmo's -- and hundreds of other startups' -- problem: Why use the new guy when I already trust the old? How can a startup oust an established company when all it has are a million dollars in venture capital and a propensity for all-nighters? More