Matt Maloney, founder and CEO of Grubhub. Adam Jeffery | CNBC

GrubHub sank 8 percent on Tuesday after analysts from KeyBanc Capital Markets said the food delivery service is struggling to keep customers amid increased competition from DoorDash and Uber. For GrubHub, "diner retention, initial diner spend and peak diner spend all appear to be deteriorating," wrote the analysts, who have the equivalent of a hold rating on the stock and a price target of $74.98. The shares reached a low of $70.36 after the report. Uber has doubled down on its food delivery service, UberEats, in recent months as it prepares for an IPO that could value the company at well over $100 billion. But the bigger share losses for GrubHub, according to KeyBanc, have come at the expense of DoorDash, which raised its last round at a valuation of $7.1 billion. SoftBank is Uber's largest investor, and the firm led a $535 million investment in DoorDash last year.

Source: CNBC

"DoorDash's share gains began accelerating in 2Q18 and show little sign of slowing," the report said. GrubHub's customer retention started falling last year. In the third quarter, the company held onto only 36 percent of diners the following period, down from 42 percent retention in the fourth quarter of 2017 and 59 percent in the first quarter of that year, KeyBanc said. The bright spot for GrubHub, which has a stock market value of about $7 billion, is that new diners are still trying the service at a fast clip. The analysts said that new diner growth averaged 28 percent over the previous four quarters, "which reflects strong market adoption of online food ordering and Grub's increased marketing efforts."