Tobacco regulations don't regulate the general case, they regulate tobacco products. I think the same will apply to social media. There's no need to define an addictive platform anymore than there's a need to define an addictive substance. Instead all that you need to define is a social media platform. And that can be expanded/adjusted as necessary.

As for Meta itself, and proving their case - I can't imagine it'll be difficult. Whatever they get in discovery + endless testimony from various people that have worked there will already likely leave it beyond any reasonable doubt.