from: http://techcrunch.com/2015/05/07/free-stock-trades/ > Tenev says Robinhood is charging a 3.5% fee to trade on margin in a private beta of the feature
It looks like they're planning to monetize via margin. That would work great if the economy was perpetually in an upswing... but I don't think they would have survived in the great recession.
Not to mention the irony of Robinhood making margin calls.