The contact list, consumer trust, and appropriate context. A third-party developer offering this has the significant challenge of making users trust them with a payment. People know Facebook and (most) trust them. PayPal or Bump are options, but are distanced from social interactions. I'd be interested to see the usage numbers from younger consumers, as I also imagine (with no data to back me up) that not many teenagers or college students use PayPal. As for cash, again it's an option, but it's pretty clear the world is going plastic. There's a reason Square is a big deal.