Its fast rise has for months been in an equally stunning retreat, which began with concerns about Groupon’s ability to lock in repeat transactions with merchants and consumers. With the after-hours trading Monday, Groupon’s market capitalization of about $4 billion was $2 billion less than Google’s buyout offer for the company in November 2010.

The collapse of the stock, along with similar performances by both Facebook and the online games company Zynga, has led to questions about whether these new fast-growing consumer Web businesses were overpromoted. Zynga, which closed Monday at $2.93, is down 69 percent from its initial offering last December. Facebook, which had its debut as a public stock last May at $38 a share, closed Monday at $21.60, down 43 percent from its offering.

While none of these companies have lived up to investors’ highest hopes, Groupon’s decline has many specifics. Some small businesses have said that they are not getting the expected repeat traffic from their Groupon offerings, giving them a net loss on the deals. Coupon buyers often fail to exercise their impulse purchases and become less inclined to purchase more.

Groupon has also faced increased competition from both Amazon.com and Google, which started its own deals business after it was rebuffed by Groupon. Both Amazon and Google are proficient in using lots of data and statistical projection to target offers.

Groupon has over the last several months hired executives from Dell, Sprint and Amazon to address its marketing problems, and has opened an office in Seattle, where Amazon is based. Those efforts are most likely too recent to have had much impact on the second-quarter performance.

Mr. Sena, the analyst, said the earnings indicated that Groupon was moving toward a greater dependence on selling discounted merchandise rather than the more profitable coupons. “I see a company making a change in its business out of necessity more than opportunity,” he said.