One are transportation companies (including companies that transport electronic communication). The key differentiator between those and social media are that the content is private. So while they have to carry anything, the content of what they are carrying is from one party to another, and the carrier may not even know what it is. So that model shouldn't apply to social media.
The other type are broadcasters, like TV and Radio. But the way those are regulated, they don't have to show anyone's content all the time. They get to choose what content is shown, but also be liable for the content being shown.
But then there are cable companies. Which carry multiple broadcast channels. And they are not liable for the content on the channels they carry, nor do they have to carry all channels. What they do have to do however is set aside some of their capacity for public access. This is how social media should be regulated.
They can choose what channels (broadcasters) they want to carry, and the broadcaster is liable for content. Funny enough, this is already how social media works! The only change would be every social media being forced to add a section that is unmoderated for "public access" in which they would not be liable for the content.
I imagine that would go about as well as you'd expect. It would be filled with child porn, hate messages, and the occasional unpopular political opinion.