Then there should be a law that requires the platform to interoperate with independent clients. You can't have both. The social network is a common good. If you want to benefit from it, then you need to treat people fairly.
Not if it's managed by a company, in which case it's a means to turn a profit. A common good needs to be managed by the community to which it's providing said good, or by an entity that's legally bound to ensure it remains "good" for the community.
The whole rationale to giving companies a charter was to enable things that are good for the community. Just because it would be profitable to kill some people, for example, doesn't mean that companies can just do it (unless they are medical insurance companies in the US).
Not if it's managed by a company, in which case it's a means to turn a profit. A common good needs to be managed by the community to which it's providing said good, or by an entity that's legally bound to ensure it remains "good" for the community.