[Jtsummers]

It's friction. If you have two entities providing the same service (money exchange), but one is easy to use and one is much harder to use, the easier one will win (all other things being equal.

Check method:

  Needed items:
    Payer - checkbook, pen
    Payee - phone

  Steps:
    Payer        Payee
    Write check
    Give check   Receive check
                 Sign check
                 Use app to photograph and submit check

Venmo method:

  Needed items:
    Payer - phone
    Payee - phone

  Steps:
    Payer            Payee
    Send $x to Payee

A similar occurrence in the financial field, mobile banking. Depositing checks via phone is easier than going to an ATM which was easier than making it to a physical branch of the right bank during business hours.

Cash is a better equivalence to Venmo, but only works when you're physically collocated. And then requires one or both parties to carry around cash (and it's not as easy to be precise, if you care about getting it right down to the penny).

[toomuchtodo]

How do you handle someone who won't install Venmo and sign up for their service? Not pay them back?

[Jtsummers]

Switch back to cash, check or another transfer mechanism. Or have them buy the next round at the bar and call it good.