"Facebook has enormous influence on users' perceptions of current events, including political perspectives," Thune wrote. "If Facebook presents its Trending Topics section as the result of a neutral, objective algorithm, but it is in face subjective and filtered to support or suppress particular political viewpoints, Facebook's assertion that it maintains a 'platform for people and perspectives from across the political spectrum' misleads the public."
It was not clear whether the alleged manipulation was still going on, as the curators quoted by Gizmodo had left Facebook.
Facebook acknowledged Tuesday that it had received Thune's letter. In a statement, the company said it "look(ed) forward to addressing his questions."
First Amendment lawyer Floyd Abrams told The New York Times that Facebook could comply with the request or refuse on First Amendment grounds.
"The notion of Congress looking into or investigating how a medium of communication decides what to say threatens on its face First Amendment rights," Abrams told the newspaper.
Harvey Silvergate, another First Amendment attorney, told the Boston Herald that Facebook's purported moderation was likely legal, though ethically questionable. It could be argued that, as a major distributor of information, Facebook could be "compared to telephone companies that are bound to neutrally relay communications," the Herald reported.
Thune's request for answers from Facebook appeared to mark a shift in position for the South Dakota senator. In 2007, he spoke out against a Federal Communications Commission rule that required broadcasters to provide equal air time to all sides while discussing controversial issues, calling it "Orwellian."
"I know the hair stands up on the back of my neck when I hear government officials offering to regulate the news media and talk radio to ensure fairness," Thune said.