If it is known that social media harms kids, then doesn't the state share some of the blame? Why is there no law?
The gears of justice grind slowly. This is by design. When you go fast, you break things. And also, no. The state didn't make them do it.
If it is not known (or only recently came to light), can you really blame the social media companies?
Yes, you could. But that's not the case. They know and have known. We've talked about that here a bunch. They willfully conduct psychological experiments on users and monitor the impact.
If the harmful effects were known to the companies and they kept it quiet, then you'd have a case, morally speaking.
That's why there's a case... no, wait, thousands of cases.
Facebook willfully psychologically manipulates people into vulnerable emotional states in order to increase engagement, they take advantage of that by knowingly spreading false information and have actually reduced the number of people they have working on reducing the false information and replaced them with automated systems which produce false positives which punish users who are conforming to their rules and standards, but seemingly do nothing to prevent actual violations.