Why have we not seen a publicly-operated social network?
It strikes me that enabling people to connect with one another is fundamental to most governments as a public service. Cities and states make and fund roadways and public squares and parks. Social networks seem like an approximate digital parallel.
Given everything that's happened with Facebook and Twitter in the last months, why are there no examples of government-run social networks?
(Or, if there are, what are they? How do they work?)
edited the title for breadth
edit 2: thanks for sounding in! A lot of great answers. To be clear, I didn't mean to present this as a leading question. I was curious about the perceived reasons from HN's audience.