Actually TikTok is now the leader with teens. [0] And I don't believe the CCP would allow a sale to a US-based company, least of all Facebook/Meta.
[0] https://www.cnbc.com/2021/11/18/tiktok-usage-topped-instagra...
[0] https://www.cnbc.com/2021/11/18/tiktok-usage-topped-instagra...
It doesn't really seem useful for sharing the same kind of information that people use Facebook for (mostly text based information, questions, etc).
They can always expand, but I feel like these social media services usually have a hard time pivoting to a different medium after their initial success.