Reddit has apologised for hosting online witch hunts as people used the social news website to try and expose those behind the bombing at the Boston Marathon.

"Though started with noble intentions, some of the activity on Reddit fuelled online witch hunts and dangerous speculation which spiralled into very negative consequences for innocent parties," Reddit general manager Erik Martin said in a blog post on Monday.

"We all need to look at what happened and make sure that in the future we do everything we can to help, and not hinder, crisis situations."

Reddit along with some of its users and moderators apologised privately to the family of a missing college student wrongly implicated during a quest to use crowd-sourcing to identify who was behind the bombing, according to Martin.

"The Reddit staff and the millions of people on Reddit around the world deeply regret that this happened," Martin said.

The San Francisco-based news-sharing platform maintained that overall Reddit served as a "great clearinghouse" for information in the aftermath of the April 15 twin blasts that killed three and wounded more than 200.

"The vast majority of these activities were positive," Martin said. "They provided a way for people to stay informed, as well as a place to just discuss, cope, and try to make sense of what happened."

Self-anointed cyber-detectives took to social media en masse in the days after the bombing, sharing and analysing photos and videos from cellphones, cameras and TV coverage of the bombing, near the Boston Marathon finish line.

Taking the lead from the official investigation, the online manhunt notably focused on people with black rucksacks, posting and highlighting photos of a string of potential suspects -- sparking warnings of vigilantism.

"I don't think we know yet whether crowd-sourced investigation like the Reddit one can work, since this is really very new," Cindy Cohn of digital rights non-profit group the Electronic Frontier Foundation told AFP news agency.

"Real law enforcement is a skill, and even they get it wrong sometimes with horrible results for those affected," she said, citing cases of mistaken identity at the 1996 Atlantic Olympics and the 2004 Madrid bombings.

"So it's important that amateurs trying to help exercise discretion and avoid jumping to conclusions."