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Facebook software spotted something suspicious on March 9.
A man in his early 30s was chatting with a 13-year-old Florida girl about sex and had planned to meet her after school the next day.
Facebook automatically flagged the conversation for employees, who read it and quickly called the police.
Officers took away 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 Mr Duncan, who praised Facebook for triggering inquiries.
Facebook is among the many companies that are embracing a combination of new technologies and human monitoring to foil sex predators, Reuters reported.
Though such techniques are available and effective, they can be expensive and may alienate some of a website's target audience - especially teens who expect more freedom of expression.
While many top sites catering to young children are vigilant, the same cannot be said for the growing number of online options for the 13- to 18-year-old age group.
Two recent incidents are raising new questions about companies' willingness to invest in safety.
Last month, the maker of a smartphone app called Skout, which is designed for flirtating with nearby strangers, admitted its use had led to sexual assaults on three teenagers by adults.
A teen-oriented virtual world called Habbo Hotel, which boasts hundreds of millions of registered users, temporarily blocked all chatting after UK television reported that two sex predators had found victims on the site and that a journalist posing as an 11-year-old girl was bombarded with explicit remarks and requests that she disrobe on webcam.