On March 9 of this year, a piece of Facebook software spotted something suspicious. A man in his early thirties was chatting about sex with a 13-year-old South Florida girl and planned to meet her after middle-school classes the next day. 

Facebook's extensive but little-discussed technology for scanning postings and chats for criminal activity automatically flagged the conversation for employees, who read it and quickly called police. Officers took control of the teenager's computer and arrested the man the next day, said Special Agent Supervisor Jeffrey Duncan of the Florida Department of Law Enforcement. The alleged predator has pleaded not guilty to multiple charges of soliciting a minor.

"The manner and speed with which they contacted us gave us the ability to respond as soon as possible," said Duncan, one of a half-dozen law enforcement officials interviewed who praised Facebook for triggering inquiries.

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