Only one in four Britons trusts social media and users would like to see tighter regulation, according to the annual Edelman Trust Barometer.
Its survey found two-thirds of Britons believe platforms such as Facebook and Twitter are not doing enough to prevent illegal and unethical behaviour, including cyber-bullying and extremism.
But faith in traditional journalism rose sharply in the last year.
Edelman said it was time social media companies sat up and listened.
“The public want action on key issues related to online protection, and to see their concerns addressed through better regulation. Failure on their part to act risks further erosion of trust and therefore public support.” said Ed Williams, chief executive of Edelman UK, a marketing and public relations firm.