Facebook, Twitter and Google lawyers defended themselves to US lawmakers probing whether Russia used social media to influence the 2016 election.
The three firms faced hard questions at a Senate panel on crime and terrorism about why they missed political ads bought with Russian money.
Lawmakers are eyeing new regulations for social media firms in the wake of Russia’s alleged meddling in 2016.
The firms said they would tighten advertising policies and guidelines.
Senator Al Franken, a Democrat from Minnesota, asked Facebook – which absorbed much of the heat from lawmakers – why payment in Russian rubles did not tip off the firm to suspicious activity.
“In hindsight, we should have had a broader lens,” said Colin Stretch, general counsel for Facebook. “There are signals we missed.”
A day earlier Facebook said as many as 126m US users may have seen Russia-backed content over the last two years.
Lawyers for the three firms are facing two days of congressional hearings as lawmakers consider legislation that would extend regulations for television, radio and satellite to also cover social media platforms.
The firms said they are increasing efforts to identify bots and spam, as well as make political advertising more transparent.
Facebook, for example, said it expects to have 20,000 people working on “safety and security” by the end of 2018 – double the current number.
“I do appreciate these efforts, but I don’t think it’s enough,” said Senator Amy Klobuchar, a Democrat from Minnesota.
Ms Klobuchar has proposed legislation that she says would make social media firms subject to the same disclosure rules for political and issue pages as print, radio and television companies.
The companies said they would work with her on the bill, but did not say they would support it.
Senators questioned whether the firms are up to the task of weighing free speech and privacy rights against concerns over terrorism and state-sponsored propaganda.
“I think you do enormous good, but your power sometimes scares me,” said Senator John Kennedy, a Republican from Louisiana.