"They make friends that way, they make connections that way. And they get inside that world, and go from there," said Lauri Stevens, a U.S. social media consultant who has been invited this week to teach police in Ottawa, Montreal and Toronto how to make better use of such sites to fight crime.
"They're seeing what people are saying about them," Stevens added. "And in some cases there are some very sophisticated investigations going on in the world of social media."
Canadian police forces have already been using such sites to some extent. This summer, for example, Toronto police used Twitter to monitor chatter from Tamil protesters who shut down the Gardiner Expressway.
Stevens said with the right software and knowledge of how to use those tools, police can mine Facebook for leads and photographs.
In addition, she said, police can use social media to connect with the community. For example, police in Toronto have used Twitter to promote themselves and Crime Stoppers in the gay community.
Ottawa police said they plan to begin using Facebook and Twitter to send and receive crime tips.
The way police use such sites has raised concerns from critics such as privacy lawyer David Elder.
"There's a real legal and ethical issue, I guess, here, about the appropriate balance between law enforcement and privacy rights. And I'm not sure exactly where that line is," he said. "It's still being set by the courts, and by the privacy commissioner."
There are currently more than 300 million Facebook users in the world, including more than 12 million in Canada.