Republican senators have accused Google, Facebook and Twitter of discriminating against conservative points of view.
The social media giants have been brought before the Senate Judiciary subcommittee this week to answer allegations of censorship and political bias amid growing discontent among conservative users.
The subcommittee is being chaired by Senator Ted Cruz, who suggested there was widespread suspicion among Americans.
Although he said "government speech police" was not the answer, he voiced concerns about the ability of tech companies to influence public debate.
"If we have tech companies using the powers of monopoly to censor political speech, I think that raises real antitrust issues," he said at a hearing, according to Reuters.
"We do have a political bias issue here," Republican Senator Mike Lee added.
Senator Mazie Hirono, who is on the panel, accused the Republican party of trying to "harass" tech companies and said the perception of bias was down to "a mix of anecdotal evidence" and "a failure to understand the companies algorithms and content moderation practices."
Facebook public policy director Neil Potts said Facebook was not suppressing conservative speech and that it "does not favour one political viewpoint over another".
Twitter public policy director Carlos Monje mounted a similar defence, saying that it "does not use political viewpoints, perspectives or party affiliation to make any decisions, whether related to automatically ranking content on our service or how we develop or enforce our rules".
In a written statement to the subcommittee, Google admitted that "sometimes our content moderation systems do make mistakes", but added that its policy was to ensure its products "serve users of all viewpoints and remain politically neutral".
