Instagram has 1B+ monthly active users (MAU) and TikTok has 800M MAU [0]. This year, TikTok has accumulated more downloads and user growth which makes it the fastest growing social network in the world. [1]
The actual competitive factor to this growth is TikTok / Douyin has access to the Chinese market and outside of China which is why it is considered as a serious competitor. Everyone else is banned.