Good question. When I worked for a defense contractor, we were highly discouraged from engaging on social media. Just one less risk to have to deal with.
While having security clearances required all sorts of sacrifices and adherence, avoiding social media ended up being a plus in the long run. To this day, I have never signed up for Twitter, Facebook, Instagram, MySpace, and the like.
The only one I have is Facebook, helps me keep tabs/contact with my former soldiers from my first command and deployment. On the down side, FB is how I found out a couple of them had committed suicide.