It doesn't. You can always split up before you grow too large, and remain private.
There's precedent: once you're a monopoly, you have extra responsibilities to the social good, like splitting up or getting regulated.
In some smaller communities, it is also has a monopoly on social media in general and instant messaging.
This is even more problematic, because such platforms have a positive feedback loop of user acquisition. Many users have no option but to use them, because all their contacts are using them.
In any case Instagram is certainly not in a monopolistic position at the moment unless you try to twist the sense of words to fit a narrative.