Many Sanders supporters never saw critiques of Sanders.
Sanders was able to broaden his appeal among liberals despite the fact that many prominent liberal pundits — including Paul Krugman, Jonathan Chait, Kevin Drum, and Jamelle Bouie were attacking Sanders for having half-baked policy proposals and an unrealistic political strategy. One big reason these attacks failed is that a lot of Sanders fans never saw them.
People on /r/politics aren’t just reading more articles about Sanders, they’re also overwhelmingly getting articles that are pro-Sanders. Articles that criticize Sanders or make the case for Hillary Clinton (or, for that matter, any of the Republican candidates) are much less likely to reach the front page.
And this doesn’t just happen on Reddit. Similar things tend to happen on social media sites like Facebook and Twitter. These sites are all about sharing with your friends. And because people tend to have similar politics as their friends, this means social media tends to reinforce what people already believe.
“It’s easier than ever to surround yourself with information that confirms what you already believed was true,” says Eli Pariser, a liberal activist who founded the social news site Upworthy. In a 2011 book, Pariser dubbed this phenomenon a “filter bubble.”
Last year, researchers at Facebook documented the extent of this phenomenon. Liberal Facebook users see more liberal articles on their newsfeeds. Conservatives see more conservative articles. The Facebook researchers argued that this wasn’t really Facebook’s fault — it simply reflects the kinds of friends people have and the kinds of articles they share. But regardless of whose fault it is, the result is the same. (Source)