People who make threats or brag about crimes on social media are on the NYPD's watch.

Assistant Commissioner Kevin O'Connor has been appointed the head of a new unit that will target online troublemakers, the New York Daily News has reported.

The new unit, part of the Community Affairs Bureau, will mine popular social media networks like Facebook and Twitter for plausible threats and evidence of crimes. It will also be on the hunt for dodgy-looking house parties advertised online.

Social media has already been a huge help to the NYPD, as in the case of Calvin Pietri back in March. When Pietri and five companions beat up and killed Anthony Collao at a house party that was advertised on Facebook, he later bragged about it there. This helped in his arrest.

While social media have been hailed as enriching freedom of expression, it has also been used for mayhem. Sections in Web sites that allow for comments are more often than not filled with a wide range of commentary. Physical fights have even been posted on the video-sharing Web site YouTube.

The latest example of social media troubles are the London riots, where perpetrators have used Twitter and Blackberry messages to mobilize.

Britisf Prime Minister David Cameron, addressing Parliament Thursday, said: "Everyone watching these horrific actions will be stuck by how they were organised via social media. Free flow of information can be used for good. But it can also be used for ill. And when people are using social media for violence we need to stop them."