They also charge no fees at the moment..
Well, ... Venmo allows the payment to be funded from a credit card. That used to be provided at par (no fee for the transfer from a card). Now they charge 3% apparently (though for new users, the first $500 of charges is free from any fees).
As far as competing with Dwolla, t here are no bitcoin exchanges or intermediaries that accept Venmo payments. So Venmo is no more like Dwolla than PayPal, AlertPay, Square, or the multitude of competing services.