A USFED-sponsored "public" space would likely end up US-centric and also would have to pander to opinions and whims of the USGOV. Not a good Twitter/FB/Reddit replacement for international users.

The first thought I had was that maybe UN should run something like this, but still, there's an issue that affects us regardless of who is running the service - social media are heavily network-effect-dependent. Unless you're willing to literally have a law against using privately-owned social media and forcing people into government-provided services, any attempt of building such a service will likely fail simply because Twitter, FB and Reddit are already there, and regular people won't have any strong incentive to jump ship.