Allegations that Facebook censored conservative news and media outlets from appearing in a section the site devotes to "trending news" sparked uproar among right-leaning groups this month. However, some observers contend, the site has a storied history of discriminating against conservatives, and the revelation about censorship should not have come as a surprise.

"We have had posts about materials contrary to Facebook's political agenda completely disappear from our main page no matter how many times we post the message," said William Gheen, the founder and president of the Americans for Legal Immigration PAC, a conservative group advocating for legal immigration. "I have almost exclusive control over these pages, so we know nobody on our team was responsible for the deletions."

Critics say Facebook has been more blatant in discriminating against gun owners, imposing a rule in 2015 that prohibited private firearm transactions from taking place through the site.

"No, not really," said Erich Pratt, the executive director of Gun Owners of America, in response to the question of whether the latest allegations of censorship surprised him. "Lots of gun owners have been disappointed with Facebook's policies towards gun rights, and unfortunately, Facebook is alienating a lot of [gun rights] supporters."

Pratt said it was "ironic" considering the level of security that Facebook founder Mark Zuckerberg employs. "He reportedly has 16 bodyguards protecting him at his home in California, and Facebook has spent almost $15 million for his security over the past three years," Pratt said.

Reports of discrimination that was more arbitrary than systematic have been rampant over the years, so much that they have become more routine than notable. When Fox News Radio's Todd Starnes set up a Facebook page in 2013, he was temporarily banned for violating Facebook's "community standards," which prohibits things like hate speech or threats.

In early 2014, former Florida Republican Rep. Allen West observed that traffic directed to his site from Facebook had dropped off substantially beginning in the first week of March that year, and he similarly wondered if that had been by design more than coincidence. "That wouldn't be nice, Mark," West said in a blog post directed at the company's CEO.

The choices involved with operating Facebook may be largely protected by the First Amendment, but that did not deter Wisconsin Republican Sen. John Thune from writing a letter to Zuckerberg in his capacity as chairman of the Senate Commerce Committee.

"If Facebook [does not] present its Trending Topics section as the result of a neutral, objective algorithm, but is in fact 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," Thune wrote in the May 10 missive, which demanded that Facebook turn over detailed information about its editorial process by May 24.

Though little precedent exists for a congressional inquiry into a website, there is at least one case in recent history. The Senate voted 96-0 in March to hold a website called "Backpage" in contempt for failing to comply with a subpoena to turn over details of its operating procedures. The charge came as a result of allegations that the website encourages child sex trafficking.

In terms of facing an inquiry from the Senate, Facebook is now in the same company as a website accused of perpetuating forced child prostitution. But because Facebook executives are likely to be more cooperative, it is unlikely the site will join Backpage in receiving a contempt charge.

After the most recent allegations of censorship, Facebook hosted a meeting for representatives of several conservative news outlets to hear about how the site's news aggregation service works. Following the event, Zuckerberg said in a statement that Facebook had been built "to be a platform for all ideas," adding, "It doesn't make sense for our mission or our business to suppress political content or prevent anyone from seeing what matters most to them."

Washington Examiner columnist Kristen Soltis Anderson, one of those who attended the meeting, said it was "positive" and called the company "well-intentioned."

Yet whatever the case may be, it is unlikely to change a belief held by many conservatives that Facebook does not tend to favor them.

"When we tried to conduct protest events against Obama in 2013 and against illegal immigration in 2014, we caught Facebook completely excluding our events from the Facebook search engines," Gheen said. "They blocked our ads and let 'trolls' violate the rules to disrupt our event pages.

"After some media broke that story, some Facebook lobbyists contacted us and claimed it was all just a 'glitch,'" he added. "But all of the glitches on Facebook point in the same ideological direction."