By Alexei Oreskovic

SAN FRANCISCO (Reuters) - Facebook Inc offered a rare peek on Tuesday at one facet of the elaborate system it uses to police its 900 million-user social network, as it attempts to keep it free of content it deems offensive, illegal or just plain inappropriate.

The company said it employs "hundreds" of staffers in several offices around the world to handle the millions of user reports it receives every week about everything from spam to threats of violence.

A detailed, and somewhat confusing, chart published by Facebook on its website on Tuesday depicts how reports of various infractions are routed through the company and lays out all the potential outcomes, which can range from an account being disabled to Facebook alerting law enforcement.

Facebook spokesman Fred Wolens said the company decided to publish the chart to provide more "transparency" about how user reports are dealt with. "There was a feeling that once users clicked 'report,' it was not immediately clear what happens," Wolens said.

The user reports are one way that Facebook maintains order on its service, where U.S. users spend more than six hours per month sharing videos and photos, playing video games and listening to music, according to research firm comScore.

Facebook, which last month had one of the largest initial public offerings in U.S. history but whose stock has since fallen below its offering price, competes with Google Inc, Yahoo Inc and other Web companies as a top online destination.

'CORE VALUE PROPOSITION'

Specialized technology and teams of Facebook staffers actively monitor the website to detect and remove content that violates its policies, such as hate speech, bullying and harassment and pornography.

Facebook's "core value proposition is that the Web is kind of a messy and dangerous place, unlike Facebook which is a safer place for you and your friends," said Ray Valdes, an analyst with research firm Gartner.

In a posting accompanying the chart on Tuesday, Facebook explained that its User Operations group comprises four teams to handle the different types of incident reports: a safety team, a hate and harassment team, an access team and an abusive content team.

A user report about a threat of vandalism for instance would be handled by Facebook's safety team, which would have the option of referring the matter to law enforcement, warning the user, or disabling the user's account.

Facebook's Wolens said the company does not disclose how often it refers incidents to law enforcement authorities. A user can also appeal a decision disabling their account to a separate group at Facebook.

But even Facebook's defenses sometimes fail, as in November when numerous users reported seeing a flood of explicitly violent images in their newsfeeds. Facebook blamed the incident on a "coordinated spam attack that exploited a browser vulnerability" according to media reports at the time.

(Editing by Muralikumar Anantharaman)