(Bloomberg) — Twitter Inc. said CEO Dick Costolo is stepping down, reflecting the social-media company’s struggle to define its vision in the months following its initial public offering.

Shares jumped in late trading, rising as much as 13% after the company announced the management change. Jack Dorsey, a co-founder, will take his place in the interim until the company can find a replacement, Twitter said in a regulatory filing. Dorsey’s appointment, effective July 1, signals that the company is focusing on improving its product after a slew of leadership changes in the past five years.

“Jack has a profound understanding of the product and Twitter’s mission, as well as a great relationship with Twitter’s leadership team, which will facilitate a smooth transition,” the company said in a statement. Costolo will remain on the San Francisco-based company’s board.

While Twitter’s ad revenue has grown substantially during Costolo’s tenure—including 71.7% in the first quarter—the social network has struggled to attract users, which is now less than Instagram. Costolo has spearheaded efforts to mitigate that effect, such as by serving marketers’ promoted tweets—its ad unit that lets an advertiser pay to ensure a particular number of consumers see its message—to two Yahoo Inc.-owned sites and apps, news reader application Flipboard and Yahoo Japan (both host sections on their sites that feature Twitter streams).