There are also goons who use social networking sites from inside join to post threats to rivals. It is also an avenue for some of them to become famous. Like the UK fugitive who kept appearing on Facebook and had a large fan following.
Found the below extract on the Washington Post
Authorities can also seek help from the Web sites. Representatives from Twitter and Facebook say they regularly cooperate with police and supply information on account holders when presented with a search warrant. Neither company would discuss specifics.
Gang use of Twitter and Facebook still lags behind use of the much-older MySpace, which remains gang members' online venue of choice.
The Crips, Bloods, Florencia 13, MS-13 and other gangs have long used MySpace to display potentially incriminating photos and videos of people holding guns and making hand gestures. They also post messages about rivals
Last week, officials in Riverside County, east of Los Angeles, announced the arrest of 50 people in a crackdown of a Latino gang they say was engaged in drug sales and hate crimes against black residents. Prosecutors say some of the evidence was pulled from MySpace and YouTube, including rap videos taunting police with violent messages.
While some members are wising up to the police attention such postings can bring, gang information remains publicly viewable online.
Dozens of Facebook accounts are dedicated to the deadly MS-13 gang, with followers from around the globe. At one site, a video displays pictures of dead members of the rival 18th Street gang, and some users have left disrespectful comments.
The toughest part about tracking someone on Twitter is finding the alias or screen name they are posting under. And many tweets are nonsensical or pointless, so cutting through the clutter can be difficult.
"It's tricky," said Los Angeles County Sheriff's Deputy David Anguiano. "If you find out what they go by, you are good to go."
Anguiano tracks the online activity of graffiti vandals - the so-called tagging crews that sometimes morph into gangs. They post tweets saying they are heading out to spray paint and sometimes post links to photographs of their work.
Often, they cannot resist bragging about their handiwork, and the electronic trail they leave is frequently used as evidence.
"They talk about it too much," Anguiano said. "You want the fame so you've got to go out there and talk about it. That's when your mouth gets you in trouble."