Secrecy News reports some interesting statistics on the non prosecution of those who leak classified information:

Between 2005 and 2009, U.S. intelligence agencies submitted 183 "referrals" to the Department of Justice reporting unauthorized disclosures of classified intelligence. Based on those referrals or on its own initiative, the FBI opened 26 leak investigations, and the investigations led to the identification of 14 suspects.

"While DOJ and the FBI receive numerous media leak referrals each year, the FBI opens only a limited number of investigations based on these referrals," the FBI explained in a written response to a question from Senator Sheldon Whitehouse (D-RI).

"In most cases, the information included in the referral is not adequate to initiate an investigation. The most typical information gap is a failure to identify all those with authorized access to the information, which is the necessary starting point for any leak investigation. When this information is sufficient to open an investigation, the FBI has been able to identify suspects in approximately 50% of these cases over the past 5 years. Even when a suspect is identified, though, prosecution is extremely rare (none of the 14 suspects identified in the past 5 years has been prosecuted)," the FBI said.