The secutrity of a government network is tenuous enough as is. Allowing access to Facebook and others is not going to improve matters in that regard now, is it? On top of all the virus concerns there's the whole publishing of sensitive information issue, and on top of that the "Bad PR" angle.
Picture this potential Facebook update by some Jet Jockey in the DoD:
User - F22FanBoi101
My Status: On stand-down after cratering my $120 Million jet - ROFL. Ejection seats FTW!!!
On a side note, amanfromMars = Iain M. Banks? Check out the last story from State of The Art and tell me you don't see the similarity.