Shares of Hertz Global Holding Inc. plunged more than 20 percent today, after the car rental company reported weak earnings and lowered its full-year profit forecast.

The company, whose brands include Hertz, Dollar and Thrifty, said its U.S. rental car revenue dropped, and it had to do a big write-down of the value of its fleet.

The weak earnings come at a time when rental car companies are facing increased competition from the likes of ride-sharing services like Uber and Lyft.  Certify.com, which provides travel and expense management services to companies, analyzed more than 10 million business traveler receipts and expenses. It found that Uber and Lyft accounted for 52 percent of ground transportation transactions in the third quarter of the year, while car rentals made up just 36 percent.