"He knew we were looking for him. But he couldn’t help himself from updating Facebook."

In Prince George’s, for example, undercover police have “friended” many gang members to help keep tabs on them and to better understand associations within the groups. Social media pages are not always available for public viewing, but users who do not properly set their security settings can leave their pages open for all to see, including the police.

Officers in the District comb sites to produce a weekly “Social Media” report for detectives on the latest information and trends related to D.C. street gangs, an ever-evolving universe of idiosyncratic neighborhood crews with assorted alliances and beefs.

“It’s like a spider web of connections,” said D.C. Police Lt. Michael Pavlik, head of the department’s intelligence unit. “You find one and track that down, and find a friend and then follow that. It’s a wealth of information, and it helps you keep up with them in a way we never imagined just a few years ago.”