The US is opening a national security investigation into TikTok, the wildly popular short-form lip-syncing and music video app that already has more than 500 million users. Why? Because TikTok is owned by a Chinese company called ByteDance. Lawmakers are worried about how that company handles the data it gathers on its 26 million US-based users, most of whom are under 25 years old. All those users produce reams of valuable personal data that Bytedance can use to improve its AI algorithms, but the Chinese government's broader AI and data ambitions have also become a national security concern for US spooks. As the alarm bells sound in Washington over yet another Chinese tech company, here's a look at where TikTok is used most, and by whom.