Considering people use LinkedIn for job networking (and sometimes in place of an actual resume), I think it makes sense to list past or active security clearances, especially in certain industries. It shows potential employers that you have or can get a clearance, which is often required prior to starting certain roles. Noting type or level of clearance probably is also helpful for lower to mid-level employees who may move between companies every few years. I don't see it as a big deal and it's reflective of the way people network and look for jobs these days. I don't think the fact that someone has a security clearance is secret (in many cases), it's more of an indication that you have passed rigorous checks. MOO.