I would simply change the law so that the behaviour of the social media corporation, search engine or whatever determines whether they are treated as a platform or a publisher. The key distinction is behaviour: a publisher edits, a platform - like the old town corner - doesn't edit.
If you are a platform, you only remove or edit material which breaks laws like child pornography, etc. You are not liable for non-criminal stuff like defamation, etc appearing on your platform. If I stand up in a bar and yell that all Jews must die (shortly before necking myself), the pub owner is not legally responsible for my hate speech. If I get into a brawl and he doesn't direct his bouncers to stop the brawl, he may be legally responsible for the assaults occurring.
If you are a publisher, you may remove or edit material for non-criminal law reasons, such as "misinformation", "hate speech", etc. You are now liable for defamation, etc, just as a newspaper would be if it'd published that same post as a Letter To The Editor.
Which you are treated as depends on your behaviour. So for example Facebook removes people's posts or edits them, and is thus behaving as a publisher. Thus if I go onto Facebook and say that John Kim is a storygamer and he sues me for defamation, Facebook would be a co-defendant in that suit.
I'd set those guidelines and then leave it up to the respective companies which they want to be. I suspect Facebook would not be able to decide, and would just collapse. Google would remain a platform, and would refrain from the editorial actions it's ventured into recently.