Facebook quietly tweaked the news feeds of 1.9 million users before the 2012 election so they would see more "hard news" shared by friends.

That change may have boosted voter turnout by as much as 3 percent, according to a little-known study first disclosed Friday by Mother Jones .

For the study, news articles that Facebook users' friends had posted appeared higher in their feeds -- the stream of status updates, photos and articles that show up when you first sign on to the site. The researchers wanted to see whether increasing your exposure to news stories shared by friends before an election would convince you to vote.

Facebook said the news stories being shared were general in nature and not political. They came from a list of 100 top media outlets from the New York Times to Fox News , according to the Mother Jones story written by Micah Sifry, a democracy activist.

Facebook shared the results with some academics, but said it did not publicly disclose the experiment until Mother Jones revealed its existence because the researchers are planning to publish an academic paper based on their findings.

A Facebook spokesman told HuffPost that the company in 2012 had tested users' news feeds "to see how some users engaged when general news stories shared by their friends were given slightly higher rankings." The company then conducted a voluntary online survey to see if their experience "made them more likely to engage in civic affairs," he said.

Voting is a core value of democracy and we believe that encouraging civic participation is an important contribution we can make to the community," a Facebook spokesman said.

Over the weekend, some Twitter users expressed concern that Facebook had conducted another experiment on its users without telling them. One Twitter user called for more disclosure and transparency.

Facebook often allows data scientists to conduct studies on its users on various topics, like a recent Valentine's Day study that analyzed whether a romantic relationship between two people on the social network would last. But the idea that Facebook has been manipulating what users see on the site has sparked controversy.

The revelation of the voter experiment comes four months after the social network was criticized for conducting a separate psychological experiment on about 700,000 users. In that experiment, which also took place in 2012, Facebook tweaked many users' feeds to omit positive or negative words, then tried to measure their emotional response -- also without their knowledge.

Facebooks voter turnout experiments have not been limited to tweaking the appearance of articles shared by friends. Since 2008, Facebook has also tried to encourage people to vote by showing a clickable button in feeds that says Im Voting or Im a Voter.

In a study during the 2010 elections, researchers displayed that button at the top of 61 million users feeds, as well as a counter showing how many other Facebook users had reported voting, and random profile pictures of the users Facebook friends who had clicked on the button.

The researchers found the buttons increased turnout by 340,000 additional votes, according to their paper , which was published in the September 2012 issue of Nature.

The voting buttons on Facebook directly influenced political self-expression, information seeking and real-world voting behavior of millions of people," the researchers said.