They aren’t a simple middle man though - they are actively choosing the rules for what content gets amplified on the platform.

They’ve done a search over the space of things that drive engagement with the site and have written a feed/content sort algorithm that optimizes for those things. They could just as easily choose to optimize for something else - like number of heart reactions only - but they choose not to because that would presumably result in lower engagement and thus less eyeball time and thus less profit.