Humans are acutely attuned to the approval of others, constantly reading cues to judge whether people agree with them, the researchers said. Active social media users get many more of these cues — like status updates, news stories people choose to share and photos of how they spend their days — and so they become less likely to speak up.

For the study, researchers asked people about the revelations of National Security Agency surveillance by the whistle-blower Edward Snowden, a topic on which Americans were almost evenly divided.

Most people surveyed said they would be willing to discuss government surveillance at dinner with family or friends, at a community meeting or at work. The only two settings where most people said they would not discuss it were Facebook and Twitter. And people who use Facebook a few times a day were half as likely as others to say they would voice an opinion about it in a real-world conversation with friends.

Yet if Facebook users thought their Facebook friends agreed with their position on the issue, they were 1.9 times more likely to join a discussion there. And people with fervent views, either in favor of or against government spying, were 2.4 times more likely to say they would join a conversation about it on Facebook. Interestingly, those with less education were more likely to speak up on Facebook, while those with more education were more likely to be silent on Facebook yet express their opinion in a group of family or friends.

The study also found that for all the discussion of social media becoming the place where people find and discuss news, most people said they got information about the N.S.A. revelations from TV and radio, while Facebook and Twitter were the least likely to be news sources.

These findings are limited because the researchers studied a single news event. But consider another recent controversial public affairs story that people discussed online — the protests in Ferguson, Mo. Of the posts you read on Twitter and Facebook from people you know, how many were in line with your point of view and how many were divergent, and how likely were you to speak up?