A start-up called Robinhood Markets is taking on the big brokerage firms with its commission-free trading app, and appears to be making headway. Since its introduction in December 2014, the app has attracted a million users and executed more than $30 billion in trades, up from $2 billion in 2015.

Despite the app’s hype and surging popularity, some industry experts question if the free-trades business model can survive, or if it will wind up joining other start-ups that have crashed and burned. The company currently makes money primarily from interest on customer cash balances.

At Robinhood, there is no minimum deposit to register an account, and there are no trading fees for customers who buy and sell United States-listed stocks and exchange-traded funds. To keep costs down, the company, in Palo Alto, Calif., takes a no-frills approach. It has no storefront offices. It does not provide research reports, analytical tools, stock screening gizmos or options trading on its platform.

“Uber opened our eyes that if you could hail a car off your phone and watch movies, why can’t I trade 10 shares of Facebook or a thousand shares?” said Howard Lindzon, a general partner of the Social Leverage Fund and a founder of StockTwits, a financial communications platform. His fund holds an equity stake in Robinhood.