There are many responses here, but I think one of the most relevant is that of natural monopolies. If you created a competitor that was somehow objectively superior to Facebook in every possible technical way, it would still almost certainly fail. Because Facebook's product isn't their software, but access to the billions of people using it. They are effectively doing little more than acting as a carrier for access to society. And you can't compete against that.
This immediately puts massive scale social media in a very unique place. By contrast, public school libraries are anything but a monopoly. If they don't offer a book you want, there are countless other ways to get it that are no less convenient, to say nothing of the digital age and searching for 'book i want pdf.' This generalizes to other scenarios as well. At one time it was believed that private businesses out be able to restrict employment or service based on things like the race or religion of their customers/applicants. But as this started to become a more substantial issue in terms of scale, society decided otherwise.