When a gang member was released from jail soon after his arrest for selling methamphetamine, friends and associates assumed he had cut a deal with authorities and become a police informant.

They sent a warning on Twitter that went like this: We have a snitch in our midst.

Unbeknownst to them, that tweet and the traffic it generated were being closely followed by investigators, who had been tracking the San Francisco Bay Area gang for months. Officials sat back and watched as others joined the conversation and left behind incriminating information.

Law enforcement officials say gangs are making greater use of Twitter and Facebook, where they sometimes post information that helps agents identify gang associates and learn more about their organizations.

“You find out about people you never would have known about before,” said Dean Johnston with the California Bureau of Narcotics Enforcement, which helps police investigate gangs. “You build this little tree of people.”

In the case involving the suspected informant, tweets alerted investigators to three other gang members who were ultimately arrested on drug charges.

Tech-savvy gangsters have long been at home in chatrooms and on Web sites like MySpace, but they appear to be gravitating toward Twitter and Facebook, where they can make threats, boast about crimes, share intelligence on rivals and network with people across the country.

“We are seeing a lot more of it,” Johnston said. “They will even go out and brag about doing shootings.”

In another California case involving a different gang, much of the information gathered by investigators came from members’ Facebook accounts. Authorities expect to make arrests in the coming months.

“Once you get into a Facebook group, it’s relatively easy,” Johnston said. “You have a rolling commentary.”

And gang members sometimes turn the tables, asking contacts across their extended networks for help identifying undercover police officers.

It’s hard to know exactly how many gang members are turning to Twitter and Facebook. Many police agencies are reluctant to discuss the phenomenon for fear of revealing their investigative techniques.

Capt. Walt Myer, director of the Riverside County regional gang task force, said gang activity often “mirrors general society. When any kind of new technology comes along, they are going to use it.”