In my little section of the DoD, DoN world, we have been enjoying FB and YT for about year, since the Commandant sent out his holiday message via FB and YT. Before that, the dreaded DoD Warning banner. As for communicating with employees, we use email, the ol telephone or sometimes if my manager needs me for something, I'll hear, "Julie!, Julie! come here a minute please."
We do not have an internal IM, unless we use our personal FB accounts, which Uncle Sam would prefer we didn't. (Not a good use of government time....use the phone or email.) The "senior" employees (who make the decisions) do not like the "interuption" of Twitter and IMs, not mention DoD has little trust for any social media, internet usage, cloud computing, et al. It's just a little quirk of paranoia they have.