From The Guardian:
Sopo, who arrived in the US in about 2003, made a living selling roses in Seattle nightclubs until, according to prosecutors, he moved on to bank fraud. He apparently drove a rented car to Mexico in late February after learning that federal agents were investigating the fraud scheme.
Investigators scoured social networking sites such as Facebook and MySpace but initially could find no trace of him and were unable to pin down his location in Mexico.
Although Sopo’s profile was set to private, his list of friends was not. Scoville started combing through it and was surprised to see that one friend listed an affiliation with the justice department. He sent a message requesting a phone call.
“We figured this was a person we could probably trust to keep our inquiry discreet,” Scoville said.
Proving the 2.0 adage that a friend on Facebook is rarely a friend indeed, the former official said he had met Sopo in Cancun’s nightclubs a few times, but did not really know him and had no idea he was a fugitive. The official learned where Sopo was living and passed that information back to Scoville, who provided it to Mexican authorities. They arrested Sopo last month.
(hat tip: Schneier on Security)