Shares of Grubhub (NYSE:GRUB) plunged about 50% over the past six months due to concerns about the food delivery service's decelerating growth, rising expenses, and market share losses to aggressive rivals like DoorDash and Uber Eats.

KeyBanc analyst Andy Hargreaves also recently warned that Grubhub's "diner retention, initial diner spend, and peak diner spend all appear to be deteriorating, which suggests lifetime value in newer cohorts is declining." Hargreaves also warns that Grubhub's active dinner growth could decelerate significantly, and that DoorDash is "gaining significant share" in the market.

Check out the latest earnings call transcript for Grubhub.

That dire warning caused Grubhub's shares to tumble, and that pressure could persist unless it proves the bears wrong. Unfortunately, it could be tough for Grubhub to allay investors' concerns about DoorDash, which is reportedly gearing up for an IPO.

How fast is DoorDash growing?

DoorDash was founded in 2013, and subsequently expanded to over 3,300 cities across the US. It was valued at $7.1 billion after its latest funding round, and its big backers include SoftBank (OTC:SFTB.Y), Sequoia Capital, and Singapore's Temasek Holdings. Last year DoorDash claimed that its service was used by 90% of the largest U.S. restaurants chains, including IHOP, Wendy's, and The Cheesecake Factory.

DoorDash charges customers service fees of $6 to $8 per order, along with a default "Dasher Tip" of 10% (which can be modified), and makes an average commission of 20% from restaurants. Grubhub lets restaurants set their own delivery fees, which generally range between $4 and $8, optional tips, and an average commission of 5% to 15% from restaurants.

DoorDash's share of the U.S. third-party food delivery service market rose from 16% in August 2017 to 31% in January 2019, according to Second Measure. During the same period, Grubhub's market share dropped from 67% to 43%. Uber Eats claimed 26% of the market in January.

Unlike Grubhub, which grew its market share through acquisitions, most of DoorDash's growth was organic. Its only major purchase was that of delivery and logistics start-up Rickshaw in late 2017. This means that DoorDash isn't spending much money integrating smaller businesses or worrying if all of its subsidiaries are on the same page.

Doordash claims that it tripled its annual sales in 2018, but it hasn't disclosed any exact sales or profit figures yet. Those figures will be revealed if DoorDash files for an IPO -- which could enable it to compete much more aggressively against Grubhub.

Is Grubhub worried about DoorDash?

Grubhub's long list of mergers and acquisitions include Seamless, MenuPages, Allmenus, DiningIn, Delivered Dish, LAbite, Eat24, and Tapingo. Those acquisitions boosted its top line growth, but they're not preserving its overall market share. Over the past year, Grubhub's growth in Daily Average Grubs (meals), gross food sales, active diners, and revenue all decelerated:

Q4 2017 Q1 2018 Q2 2018 Q3 2018 Q4 2018 Daily Average Grubs 34% 35% 35% 37% 19%* Gross Food Sales 39% 39% 39% 40% 21% Active Diners 77% 72% 70% 67% 22% Revenue 49% 49% 51% 52% 40%

During last quarter's conference call, Grubhub CEO Matt Maloney didn't mention DoorDash, and declared that the competition "really hasn't slowed our growth" or increased its customer churn rate.

However, Grubhub's slower growth in active diners indicates that it's struggling to grow its customer base (and is possibly running out of smaller companies to acquire), and its forecast for 31% to 42% sales growth this year would represent a significant slowdown from its 47% growth in 2018.

Furthermore, Grubhub's heavy spending on big marketing campaigns, acquisitions, partnerships, and the expansion of its logistics network caused its net income and EBITDA levels to plunge during the fourth quarter.

Q4 2017 Q1 2018 Q2 2018 Q3 2018 Q4 2018 GAAP net income 293% 74% 104% 75% (110%) Non-GAAP net income 68% 88% 99% 72% (47%) Adjusted EBITDA 45% 51% 61% 41% (26%)

Grubhub expects its EBITDA to rise just 1% to 3% this year, compared to 27% growth in 2018, so we should assume that tougher competition is forcing Grubhub to ramp up its spending to prevent further market share losses. However, the expansion of certain services, like loyalty and payment services, could also lock in its existing restaurants.

Will investors get a nasty surprise in May?

Grubhub's stock tumbled after its fourth-quarter earnings in February, and it could disappoint investors again with its first-quarter report in May. Grubhub's management is oddly dismissive of the competition, but investors should identify DoorDash as a disruptive threat that could steal away its customers and restaurants.