As long as there's operational independence and no bias for particular nodes, what's the issue with Facebook breaking itself up and federating the Baby Books with something like Mastodon?
Because the purpose of this law is to suppress the right of online platforms to moderate certain forms of speech, and to compel them to publish that speech against their will.
If Facebook or any other platform breaks up in order to avoid that, the government will just rewrite the law in such a way that it still includes them.
It's harder to regulate a bunch of protocols and distributed entities. Regardless of purpose, regulation which only kicks in at 50m users seems a solid incentive to force networks to remain under that size, and that seems socially beneficial to me after the past decade. Scale benefits distributors but destroys diversity.
If Facebook splits into 50 Baby Books, Zuckerberg's overall wealth likely increases, yet power is distributed. Ma Book could try to control the protocol, or a reference implementation, but they would need to do so in the open, and offer up transferable user data, which would mostly solve the issue with asymmetric access to "their" user data. Each Baby Book would need to compete on design, features, moderation/labeling/verification/filtering, branding, ad tools, etc. and that would be just fine.