Snap Inc. CFO Tim Stone is resigning after eight months in the job, another sign of volatility at the social-media company that sent shares tumbling in extended trading.

His sudden departure, following several years of frequent executive turnover, adds to uncertainty surrounding the young company's future as it struggles to expand the user base for its Snapchat mobile photo-sharing app. Snap shares, which have fallen more than 50 percent in the past year, dropped 7 percent after the close of regular trading in New York yesterday, following the announcement.

Stone, who came to Snap from Amazon.com Inc., was seen as an important veteran executive hire after a lot of tumult. Since the company's March 2017 initial public offering (IPO), most of Snap's top executives have either chosen to leave or been pushed out, including the heads of legal, strategy, product, engineering, and sales. CEO Evan Spiegel's handling of the departures has in some cases caused friction. When replacing Chief Strategy Officer Imran Khan last year, he offered the job to one person, then changed his mind and offered it to someone else.