Arguable IMO it could be argued for civic engagement and discourse and education. That’s the charter as I understand for NPR, I don’t see why a public social network couldn’t fall under that.

Misinformation on social media exists today, I don’t see why we couldn’t argue as a society that public discourse on the network couldn’t be perceived as a way to educate the public.

As far a infrastructure, just limit content type and posting limits (time gate posts on topics, or limit 1 post per day, etc.)

Companies would get public data to be trained since everything would be CC licensed. And a contract to be won for reserved services.