Shares of online auction leader eBay (EBAY - Get Report)   tumbled 8.9% Friday to close at $28.75, their lowest level since December 2016.

The San Jose, Calif.-based company took a hit after PayPal (PYPL - Get Report) CEO Dan Schulman outlined a study indicating the harm that eBay's decision to drift away from its trusty payment partner will bring. EBay first reported its plan to cut ties with its long-time strategic partner PayPal in January. Since then, shares have lost nearly 25% of their value.

"The sellers that eBay and PayPal share demand to have PayPal," Schulman explained in an earnings presentation Thursday night. "Their sales actually depend on it."

He backed his claim up by citing a market-research report commissioned by PayPal that revealed consumers on average are "54% more willing to buy when a merchant accepts PayPal."

Adding to the concern for eBay, the study states that 59% of PayPal users have abandoned a transaction on eBay simply because PayPal wasn't a checkout option.

"We've got 254 million customers on our platform right now," Schulman said. "Imagine if almost 60% of them would abandon a sale because PayPal Checkout wasn't available."

Analysts have picked up on this concern, adding to market anxiety about eBay.

"We are lowering our rating on eBay shares to 'Hold' from 'Buy' based on PayPal's earnings-release disclosure of weak eBay third-quarter gross-merchandise volume trends," Stifel analyst Scott Devitt wrote Friday morning. He warned that the transition to alternative payment platforms is likely to lead to a "prolonged period of compressed valuation for eBay shares."

To read more from Devitt's report and hear his recommendations on where else to put your money in e-commerce, check out our Stock of the Day coverage on Real Money, our premium site for active investors.

(This article has been updated with closing stock prices.)

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