Federal agencies are nibbling their nails on the side-- intrigued, willing to dip a toe, but still holding back from true social engagement.
While the resources are there, I have a hard time thinking of federal program that really engages (with each other, customers, partners, investors, anyone), as opposed to, just pushing information via Facebook and Twitter. On the proactive side, GSA keeps up a reference site focused how to "deliver a better customer experience" through social media. On the reactive side, change.org hosts literally dozens of petitions with federal flavor. Many of these have a legislative objective-- like this one for ending the marijuana prohibition or this one for anti-bullying.
Part of the problem is that federal managers underestimate the potential influence AND risk exposure social networks bring to them. The other well-known issue is that there are so many controls on external communications that it is difficult to get approved juicy messages real time. But the root is fearing loss of control, I think. Yet tremendous potential exists for the fearless, confident fed.