I am not surprised of the little traffic on your wiki. It is hard to make people change their way of doing their work.
I am actually trying to convince a small group in my agency (FDA) to start using a wiki as our shared work space. Will see how it goes. I think it works better if you start with small groups that have common goals/interests and then scale it up. You may need first to identify potential early adopter and they will then convince other people in their circles.
Nonetheless, there are very successful examples using wikis at very large federal agencies. The DoS, DoS, and the CIA have created agency-wide wiki and seem very successful. These agencies created Dilpopedia, the DoD Techipedia and the Intellipedia.