[dumbfounder]

I'm assuming you mean available to the general public, run by the government? There probably are smaller, niche social networks for gov employees to collaborate. If there aren't there probably should be.

But to answer the assumed question, it's because there is no practical way they could compete with private companies. Sure, theoretically they could dump tons of money into developing cool new features, slick UIs, and obtain top talent. They would also need to spend billions to crush or acquire upcoming networks to maintain their dominance (Whatsapp, Instagram). Or legislate them out of existence. In practicality there is no way this would (or should) ever happen.

[nrvslghtnng]

The "they'd never develop great features" argument is an interesting one, as I think most social networks can contribute most of their success to the network effect itself. Once that positive feedback loop gets churning, it's hard to turn it around.

Twitter's a neat counterexample here, I think. On the front-end, at least, the features released by Twitter in the last few years haven't left me awestruck. Yet it has become a seemingly self-sustaining space on the internet for public discourse.