Regardless of how good you believe your argument is philosophically there is no law that supposes they are limited to choosing supporting unrestricted speech or shutting down and any attempt to create such a law would in fact be a violation of their legally defined rights.
The change you desire would almost certainly require a constitutional amendment to allow the government to compel the speech of a corporation in order to protect the speech of their customers. It would be complex, challenging, hard to predict, and almost certainly impossible even if people on average agreed it was needed.
You would have an easier time forcing payment processors, cdns, hosts, not to discriminate against alternative social networks. They at least ARE infrastructure. The challenge with those alternative networks is the only content that absolutely must use it is often odious leaving such networks with little of the desirable often uncontroversional content produced by the rest of the population of users and lots of the bullcrap nobody else wanted.
For example technically voat is more "free" than reddit but a brief look at the frontpage suggests that this freedom is mostly used to be the kind of offensive jerks nobody else wants to be around.