Kevin Mayer, the Disney executive who oversaw the launch of the streaming service Disney+, is leaving his post at the entertainment giant to become CEO of the fast-growing social media app TikTok.Disney veteran Rebecca Campbell will succeed him as chairman of director-to-consumer and international."I am very proud of what our extraordinarily talented direct-to-consumer and international team has accomplished in creating and delivering a world-class portfolio of streaming services, particularly Disney+," Mayer said Monday in a statement.Mayer joins TikTok at a pivotal moment for the app, which is owned by Chinese conglomerate ByteDance.