It has dominant market share of photo-centric social platforms.
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.
The definition of a tiny specific monopoly within a niche is meaningless. If you had 30 different factors to describe exactly what each platform does then each of them would be a monopoly in themselves.
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.
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.