I haven't heard of it as an internal tool. But as you said lots of agencies use it and a large part of their audience is their own employees
Unfortunately, my agency doesn't use social media at all to communicate/collaborate with employees. They are exclusively used to interact with external stakeholders. In fact, most social networks are still blocked from our government-furnished equipment. Most of us have learned to adapt and use our personally owned devices to access social networks. I'm a big user of Twitter, but many of my colleagues have not yet discovered the wealth of information available by this great aggregator of news feeds.
I still hold out hope that someday, our enlightened leadership will realize the value of social networks for interacting with perhaps an agency's most important stakeholder - its employees. Until then, we depend on mavericks and outliers like yours truly.