It only appears unsolvable because you've presumed that social media should exist in its current form. Yes, algorithms can't match the scale of the bad content before we hit AGI. But that problem only exists because we have for-profit companies hosting way more content than they can afford to police on the thin margins ads provide. (Twitter, for example, makes about $1 per user per month.)

Prior to the late 2000s, this problem didn't exist. In alternate universes, it surely doesn't; there are many ways this could have gone.