Social media and big tech do in fact represent a monopoly, but not in terms most people are familiar with. Instead of acting only as an economic monopoly controlling market share, big tech is also a political monopoly controlling the majority of communication platforms. If only one political and social ideological group dominates every major social media and digital information outlet, this in my view represents a completely unbalanced power dynamic that does indeed threaten the free speech rights of the populace.
Corporations ARE NOT free market structures. They are, in fact, government chartered and government protected monopolies. They are SOCIALIST creations, not free market creations, and therefore they should not exist in a free market society at all.
Instead of challenging the corporate model in the Supreme Court, an easier option would be to simply take away all welfare and tax incentives for any big tech companies that refuse to allow free speech on their platforms.