What exactly did your rejection letter say?  If this was for the CIA, NSA, DIA, or another Intelligence Agency, it was probably one of two things:

1. The position you were recruited for is no longer available.  Maybe they filled it with somebody else or don't have the funding to hire you.

2. Your foreign contacts on facebook dinged you.  As silly as this sounds, the Intelligence Community looks for any small thing to DQ you for.  If your letter said you were "unsuitable", this is why.

I hate to say it, but sometimes it is best to lie for these jobs.  The investigators only know what you tell them (like in your SF-86 or coerced polygraph confessions), or what is on file (if you filled for bankruptcy, criminal convictions, mental health counseling that you admitted to on your SF-86 and signed away your privacy rights...by the way, health info is protected by some privacy act so you should not even disclose this, etc.).They do not look at your Facebook friends for the background check, unless you have your privacy settings all open then they can see your stuff at will.  It is also best not to disclose your naughty deeds that you've done that nobody knows about.  If you banged a hooker overseas, do not tell the federal investigators.  They will never find out, they are not magical brainreaders.

Sorry to say, but those foreign contacts probably did you in.  Especially if they were from unfriendly territories (middle east, russia, china, etc.).