Generally, to be interesting for anti-trust legislation, a company does not need to have absolute control of the market. It needs to have just enough not to worry about competition. Which is a position where Twitter/Facebook are. They tend to acquire all interesting startups that could threaten them.

Define “the market” though. Why is it that tik tok can not exist and suddenly become a huge phenomenon in just a few years and Facebook can do nothing about it?

That to me clearly says that FB doesn’t have a monopoly unless you define the market to be so tightly focused on what they control it becomes tautological