Because the government is the host, they can't censor anything on the platform, unlike private social networks. The moderation policy would effectively be the content of the first amendment, barring actual illegal content.
Call it publicsquare.gov
Anything you posted on facebook.gov would be instantly available to advertisers AND the government so a huge segment of the population would refuse to use it especially young people. It would probably end up with a mixture of a minority of socially minded people and people so odious they would be censored anywhere else. See the kind of folks you have on the conservative reddit clone. As soon as people conspiring on facebook.gov are guilty of something really terrible like say a mass shooting there will be calls to implement some sort of additional content controls and we now have 2 facebooks one paid for by ads another by our tax money.
Instead of building a hypertext website they could finance an ARPA-E program to develop new federated content publishing protocols. Then have the Department of Energy build out and maintain the initial cloud infrastructure as a public utility.
Any user files stored on network slices have the same legal and privacy protections as mail held at the United States Post Office, and people are free to store encrypted files using whatever encryption scheme they want in the same way that they are free to send pages of gibberish through the mail.
If DoE builds out a cloud network then DoD could also lease a portion of the infrastructure for JEDI.
Trolling categorised as "obscene" would be removable, but otherwise may be protected by the First Amendment. Also, the fact that every content moderation decision may be subject to litigation would make it extremely difficult for the government to meaningfully moderate content.
Private platforms, not bound by the First Amendment, and allowed to moderate without assuming liability by Section 230, wouldn't suffer these problems
[1]: Via White Buffalo v. UT Austin
In a similar vein with the fact that it's illegal to yell out "FIRE!" in a crowded theater unless there is actually a fire, they could make it illegal to advertise private businesses and make posters directly legally responsible for what they post.
If this does come about though, I hope they prohibit businesses and "Media Groups" or whatever from creating accounts or at least lock them into a walled garden that you have to voluntarily enter, segregating them from the general populace.