I’ve blogged before about how social media has had a major (positive) impact on political and social movements, such as the “Arab Spring” uprisings and the democracy movement in Moldova. But recent news out of Mexico shows how the same social media can contribute to additional fear and violence in a society that already has more than its share of drama and danger.

In recent weeks, CNN has reported that social media is causing citizens living in the regions of Mexico beset by dangerous drug cartel activity to be injured – or even killed. In the border city of Nuevo Laredo, the bodies of a man and a woman were found hanging from a highway overpass, with threatening notes nearby promising a similar fate to other so-called “Internet snitches.”

According to the CNN news story, the two people were killed for messages they had posted online pertaining to drug violence in areas of Mexico where the professional news media are no longer able to do their job.

Because drug trafficker threats have essentially silenced reporting activity in these border regions, the local citizens have resorted to filling the information gap by using social media like Twitter and Facebook to convey the latest information to their fellow citizens.

The notes affixed to the dead bodies in Nuevo Laredo appear to have been left by members of two notorious drug cartels that are intimately engaged in the region’s bloody turf wars.

The killings point to a dangerous new front that’s opened up in the drug wars: In the absence of credible news reporting, many residents of the borderlands have turned to social media platforms for learning and sharing information. Using #hashtags that tie Twitter posts together has become an important “sorting” mechanism by which postings from individual Twitter accounts can be bundled into a sort of jerry-rigged news service. Many examples of “news report bundling” exist, such as for cities like Monterrey, Veracruz, Saltillo and Reynosa.

Andrés Monroy-Hernández of MIT’s Media Lab has studied this phenomenon, and declares that these ad hoc news bureaus have been effective.

“Most of the information is reliable, and the information that is not often goes ignored … [these bureaus] serve as curators and do a decent job at it,” the Mexican native asserts. He also points out that about half of the Twitter messages are actually retweets, meaning that people are cooperating with one another in spreading the information.

But the startling events of last week remind us that local residents who are using social media to navigate the chaos of the drug wars are themselves becoming targets in the drama.

Even the Mexican government is in on the action. Recently, it charged two Veracruz citizens with “terrorism and sabotage” for passing along rumors of a pending cartel attack on a school that resulted in an outbreak of panic at the school property.

The Mexican government’s action set off a wave of criticism from all sides. Amnesty International went on record stating that the drug war “creates a climate of distrust in which rumors circulate on social media as people try to protect themselves, because there is no reliable information available.”

The chaos that is enveloping Mexico – and the tragic consequences that stem from it – seem hard to imagine happening so close to the U.S. border. It’s also a reminder that the “brave new world” of social media can harbor grave dangers in addition to great promise.

And in this case, it can even get you killed.