London police say they used phones confiscated from people they'd arrested to monitor BlackBerry Messenger for signs of riot-related activity.
According to Assistant Met Police Commissioner Lynne Owens, the method bore results.
"Through Twitter and BBM there was intelligence that the Olympic site, that both Westfields [shopping centres] and Oxford Street were indeed going to be targeted," she told the home affairs select committee. "We were able to secure all those places and indeed there was no damage at any of them."
Ms Owens had a team ploughing through an "overwhelming" pile of "chitter-chatter" on social networks trying to track down self-appointed organisers of street riots around the capital.
Testifying before MPs on the home affairs committee, acting Metropolitan police commissioner, Tim Godwin, said access to BBM was a handy "intelligence asset", along with what could be gleaned from Twitter.
Asked what transpired after BlackBerry maker RIM offered to help police access its encrypted Messenger service, Godwin declined to comment. Asking to "plead the fifth", he said: "I would rather not answer that question as it is an investigative strategy."
Godwin confessed that police officers had wanted to shut down social messaging sites, such as Twitter and Facebook as well as the BBM service, but it took them a while to realise they didn't have the authority to do such a thing.
"We did consider seeking the legal authority to switch it off. The legality is questionable, very questionable," he said.
Some Tories, including Prime Minister David Cameron, have argued that social networks should sometimes be taken offline to combat civil disorder.