I'm guessing many people working in security don't have LinkedIn profiles. It's not like you want to advertise a stint in Fort Meade, and then a list of people someone might contact to get access to you, or pull some social engineering. Or advertise your TS/SCI in your profile.
There's more and more places where the less visible presence online you have, the more you're a good fit for the position.
There's more and more places where the less visible presence online you have, the more you're a good fit for the position.