Senator Mark Warner told the hearing that social media firms "were caught flat-footed by the brazen attacks on our election" and questioned whether Silicon Valley is capable of confronting the problem of foreign influence.
The US Justice Department said on Wednesday it would probe social media giants over concerns over competition and "stifling the free exchange of ideas".
Dorsey was headed later Wednesday to a House committee focused on GOP complaints that social media companies have shown evidence of bias against conservatives.
The threat of potential federal intervention came just as Twitter CEO Jack Dorsey and Facebook COO Sheryl Sandberg were testifying before Congress about their efforts to root out false news and foreign meddling in U.S. elections.
"Unfortunately, what I described as a 'national security vulnerability, ' and 'unacceptable risk, ' back in November remains unaddressed", Senator Richard Burr, the committee's Republican chairman, said. They have always censored speech to attract advertisers and prevent harm against users, whether it's pictures of female nipples, bullying and harassment, or terrorist propaganda. "I'm not even sure it's trending in the right direction", he added.
"We did take way too many hours to act, and we are using that as a lesson to help improve our systems", Dorsey said.
Naturally, Dorsey said Twitter doesn't "feel great about this", and that the company can't use its scale as an excuse.
Dorsey testified with Facebook COO Sheryl Sandberg about election meddling and abuse on their platforms.
Twitter's Dorsey was to follow his Senate testimony with an appearance at an afternoon hearing looking at that issue in the House of Representatives.
"We need to seriously think about whether the time has come for these companies to abide by new transparency obligations", Pai said in a blog post.
The companies have conveyed to the commission that all sponsored content in favour of a political party, political leader or candidate will flag the concerned sponsor and the amount paid for posting the content on Facebook, WhatsApp (owned by Facebook), Google or Twitter.