Robinhood is bracing for a slowdown in retail trading that may last until 2022.

The online brokerage has been among the biggest beneficiaries of the flood of retail investing over the past 18 months, with its equities arm driving the broader wave until the second quarter, when cryptocurrency trading took off.

Now, following a third quarter in which transaction based revenues—particularly in crypto—dramatically slowed, Robinhood warned that seasonal headwinds and a decline in trading during the three months ending Sept. 30 may persist well into the fourth quarter.

"We expect our growth not to be linear," Robinhood CFO Jason Warnick said on a conference call with Wall Street analysts. "It should ebb and flow with market conditions."

Spooked by the downbeat comments, investors sent the company's shares tumbling more than 8% in after-hours trading on October 26.

Barring any changes to the market environment, the brokerage now forecasts fourth-quarter revenues of no higher than $325 million, or just a slight gain over the $317.5 million it collected during the same period last year. Robinhood additionally said it expects around 660,000 newly funded accounts in the final three months of 2021, roughly the same gain as in the third quarter.

The downbeat guidance comes months after a buzzy initial public offering on Nasdaq in July. Robinhood had total third-quarter revenues of $365 million, a 35% gain from the $270 million seen a year earlier. Transaction-based revenues, which accounted for the biggest part of overall sales, grew 32% to $267 million. That number was primarily helped by growth in customer cryptocurrency trading, which saw related revenues jump 860% year over year to $51 million.

Robinhood's net loss of $2.06 per share in the third quarter was not as bad as Wall Street feared, as analysts had a median estimate for a net loss of $2.71, according to S&P Global Market Intelligence. Meanwhile, monthly active users swelled 76% to 18.9 million from the prior-year period.

And yet, when compared to the second quarter, Robinhood's third-quarter results paint a vastly different picture.

Cryptocurrency transaction revenues fell 78% sequentially, while equities and options ticked slightly lower. Net cumulative funded accounts fell slightly to 22.4 million. And average revenues per user were nearly halved from $112 in the second quarter to $65 in the third quarter.

Executives of the high-flying brokerage did warn ahead of time that a dip in activity was imminent for the third quarter. However, investors were still surprised by October 26's results.

"We are still very early in our journey," cofounder and CEO Vlad Tenev told analysts. "Historically, our growth has come in waves."