When I was in the Marine Corps, there was a reservist who had been activated to serve in the Sensitive Compartmented Information Facility (SCIF) where we worked, and therefore was required to have a Top Secret SCI clearance.
However, a problem arose when he decided to post his resume on job sites such as Monster.com, and listed his security clearance level on the resume.
It makes sense, right? You want to find a job and know that your security clearance can help you. Why would this be a problem?
The issue comes from the fact that you are letting the world know that you have a Top Secret security clearance, because anyone can find your resume on those job sites.
Having access to national security information -- especially at a level where unauthorized disclosure could cause grave damage to national security, as is the case with a Top Secret clearance -- means nefarious individuals might be on the lookout for ways to take advantage of you.
It might be access to a SCIF, information on classification codes or security procedures, and much more.
But is this a security violation? No, not according to the NSA and other agencies' resume guidelines.
While the person in my command got his wrist slapped, he wouldn't seem to be in the wrong (unless he had been told specifically not to post the information -- it is the military after all and, as they love to say, you pretty much belong to them).
The key is to use your judgment, and not put yourself at risk.