Kevin Systrom and Mike Krieger, the co-founders of the photo-sharing app Instagram, have resigned and plan to leave the company in the coming weeks, adding to the challenges facing Instagram's parent company, Facebook.

Mr. Systrom, Instagram's chief executive, and Mr. Krieger, the chief technical officer, notified Instagram's leadership team and Facebook on Monday of their decision to leave, said people with direct knowledge of the matter, who spoke on condition of anonymity because they were not authorized to discuss the matter publicly.