The number of organizations reporting diversion of funds for private benefit is small in relationship to the nonprofit sector as a whole.   Some would extrapolate from this number of known cases to conclude the numbers that are not caught are vast. 

Below are some articles on this topic.

Inside the hidden world of thefts, scams and phantom purchases at the nation’s profits.

Link: http://www.washingtonpost.com/investigations/inside-the-hidden-world-of-thefts-scams-and-phantom-purchases-at-the-nations-nonprofits/2013/10/26/825a82ca-0c26-11e3-9941-6711ed662e71_story.html

Despite High-Profile ‘Three Cups’ Controversy, Nonprofit Fraud Isn’t Widespread

Link: http://philanthropy.com/article/Nonprofit-Fraud-Isn-t/127268/

How to Steal from a Nonprofit: Who Does It and How to Prevent It

Link: https://nonprofitquarterly.org/2007/12/21/how-to-steal-from-a-nonprofit-who-does-it-and-how-to-prevent-it/

An Investigation of Fraud in Nonprofit Organizations: Occurrences and Deterrents

Link: http://nvs.sagepub.com/content/36/4/676.short

Unreasonable Compensation Is A Diversion of Funds

Link: http://www.thenonprofittimes.com/news-articles/unreasonable-compensation-is-a-diversion-of-funds/