Facebook posted its biggest one-day drop since May 29 amid investor concerns the company’s earnings report next week will point to slowing sales growth.

The stock fell 8.1 percent to $28.25 at the close in New York. It has slid 26 percent since the Menlo Park, Calif.-based company sold shares in an initial public offering in May.

Some investors may be selling the stock ahead of Facebook’s earnings on July 26, said Laura Martin, an analyst at Needham & Co. in New York. Facebook is expected to report second-quarter earnings, excluding some items, of 11 cents a share. An earnings report from Web portal Yahoo! today will probably give Facebook investors an early glimpse into Internet advertising in the second quarter, she said.

“Some of the stock weakness could be in anticipation of weaker earnings,” said Martin.