The different varieties of intelligence that the U.S. government is involved in offer some indication of the size of the U.S. intelligence effort. There are sixteen U.S. intelligence agencies. The U.S. intelligence budget (excluding the Military Intelligence Program) in fiscal year 2013 (the most recent year for which I could find unclassified data) was appropriated as $52.7 billion. It has almost certainly grown in recent years. How many people are involved is far from clear. In 2010 (again, the most recent figures I could find) the intelligence community reportedly included 854,000 people holding top-secret clearances.
The U.S. intelligence community, in other words, is a silent giant surveilling other nations so as to protect us. We almost never hear anything about the intelligence apparatus in the press. The lack of public information is a testament to the integrity of thousands of intelligence operants who are devoted to remaining silent lest their ability to detect what our enemies are up to would disappear.
As a former member of the brotherhood, I bow in respect before them.