Trust in information on the web is being damaged by the huge numbers of people paid by companies to post comments online, say researchers.
Fake posters can "poison" debate and make people unsure about who they can trust, the study suggests.
Some firms have created tens of thousands of fake accounts to flood chat forums and skew debate.
The researchers say there are reliable ways to spot fakes and urge websites to do more to police users.
The researchers from Canada and China say paying people to post comments is an "interesting strategy in business marketing" but it is not a benign activity.
"Paid posters may create a significant negative effect on the online communities, since the information from paid posters is usually not trustworthy," they wrote....read more