Takeout stocks plunged on Friday after Amazon announced an investment in Deliveroo—highlighting the promise and risk of investing in the meal-delivery space, which is likely to be reordered by deals.

Takeout stocks plunged on Friday after Amazon announced an investment in a fast-growing meal-delivery startup. But investors may need to look elsewhere for the real victims of all the cash flowing into a sector that epitomizes the promise and profligacy of Silicon Valley.

Deliveroo, founded by two Americans in London in 2013, announced on Friday a $575 million funding round led by Amazon. Shares in Just Eat, a pioneering U.K. website for ordering takeout food that now competes with Deliveroo, fell almost 10%. Shares in Delivery...