Senator Al Franken asked Facebook why payment in Russian rubles did not tip off their security teams to suspicious activity.
“In hindsight, we should have had a broader lens,” said Colin Stretch, who is acting General Counsel at Facebook.
Senator Amy 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.