Not many analysts or investors expected TD Ameritrade (Nasdaq: AMTD) to post great numbers – the analyst target was 3 cents less than last year's third-quarter earnings.

E*TRADE's (Nasdaq: ETFC) slightly profitable results and Charles Schwab's (NYSE: SCHW) fine but not spectacular results raised the bar a little, but not much.

Still, TD Ameritrade disappointed. Profits for the third quarter fell by 27% as the company earned 20 cents a share on revenue of $608.8 million. Analysts were looking for 23 cents per share on $619.2 million in revenue.

TD Ameritrade and its competitors have been hit hard by the economic downturn. Near-zero interest rates have made money market fund rates so low that brokerage customers could suffer a loss just by holding cash in one of these funds. As a result, these brokerages have been forced to waive fund fees that in the past have served as important streams of revenue.

While TD Ameritrade was also hurt by low trading volumes, the company actually saw a 3% volume increase in the month of September over the same period last year. However, last week was the 24th consecutive week in which money has flowed out of domestic equity mutual funds. The total amount of money redeemed by mutual fund investors this year now stands at $81 billion, showing that retail investors remain ambivalent to the stock market.

While TD Ameritrade was hurt by many of the same issues affecting its peers, the company's bottom line was also hurt by an unexpected 4% increase in expenses. Most of the added expenses can be attributed to the company's acquisition of options-oriented brokerage thinkorswim and additional advertising costs.

However, for the quarter, client assets did rise 17%, and the company added $6 billion in net new assets. Chief Financial Officer Bill Gerber suggested, "With interest rate sensitive assets at an all-time high of $66 billion, we believe that we have a significant earnings upside once rates rise."

While the earnings upside may be significant, the key question for investors and for Mr. Gerber is when will interest rates rise?

Andrew Bond owns no shares in the companies listed. Charles Schwab is a Motley Fool Stock Advisor recommendation. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Fool has a disclosure policy.