It clearly states on the site they make money of every text message sent.

edit: from the info pages

"We charge a transaction fee every time you receive money. Sometimes, the person sending you money can pay for the fees associated with the transaction. Those fees are outlined on our Venmo Fees page."

There's also a premium service for businesses.

http://info.venmo.com/pricing/pricing-plan-comparison/

But the person who submitted the story says otherwise:

http://news.ycombinator.com/item?id=948793

As of right now they are not charging... it is open beta... but i'm sure that will change.

That says a lot about startups now a days.

They could probably get away with charging 1% or 2%, but it looks like they don't take any transaction fees.