Growing fortune of world's most valuable startup by Venus FengSoftware engineer Zhang Yiming started out producing apps for sharing jokes before focusing on news aggregation. That pivot proved lucrative.The 35-year-old founder of Bytedance Ltd. is worth about $13 billion, according to the Bloomberg Billionaires Index, making him China’s 9th-richest person and one of the fastest in modern times to amass a mega-fortune. The business, founded in 2012, has more than 1 billion active monthly users across eight mobile apps, including a news aggregator powered by artificial intelligence and a video-sharing platform.His rapid wealth accumulation is a sign that China hasn’t lost its knack for creating mega-rich company founders despite a slowing economy. It also helps explain why authorities seem to be taking a more tolerant stance toward a corporate structure favored by the country’s technology tycoons, most of whom have chosen to list their businesses overseas.Zhang’s fortune is harder to calculate than the founders of Baidu Inc. and Tencent Holdings Ltd. in part because his company isn’t yet public.