The only issue is that Facebook's scope is transnational, so you can't solve this locally or nationally. The same argument probably applies to similar platforms that facilitate "public discourse."
Perhaps something under the UN umbrella will eventually emerge to fill this vacant space—not unlike how nation-states nationalised railways in the nineteenth century?
Could not disagree more. I am not a Facebook user. While I have benefited much from education, health, and roads, and would stand to lose a lot were they not present even if I chose not to use those services. As a non-Facebook user, my life has not been negatively impacted by not using it, nor has it been positively impacted by other's using it.
The value of this service is so low that I would absolutely be opposed to the state getting involved in providing Facebook alternatives, and would really not want my taxes going towards it. Let those who value it pay for it (whether financially or through their data).
Just to give a micro example, last month parents at my kids' primary school used a Facebook group to challenge (albeit unsuccessfully) top-down changes in the school's governance. I'm talking about that kind of thing, not holiday photos.
Care to reassess your proposal? :)