Most retweeters and Facebook reposters aren't informing, or even arguing. They are using headlines the way one might use a bumper sticker: to express who there are and bond with others.
From a user’s point of view, every share, like or comment is both an act of speech and an accretive piece of a public identity. Maybe some people want to be identified among their networks as news junkies, news curators or as some sort of objective and well-informed reader. Many more people simply want to share specific beliefs, to tell people what they think or, just as important, what they don’t. A newspaper-style story or a dry, matter-of-fact headline is adequate for this purpose. But even better is a headline, or meme, that skips straight to an ideological conclusion or rebuts an argument. [(Source)](http://www.nytimes.com/2016/08/28/magazine/inside-facebooks-totally-insane-unintentionally-gigantic-hyperpartisan-political-media-machine.html?_r=0)
Much position taking is not substantive, but presentational. See Identity More Than Policy
In Partisan Sorting people first sort into parties based on belief, but then have beliefs molded by party.
Amber Case's Templated Self is an attempt to understand how UI mediates expression of identity.
Identity Headlines combined with homogeneous groups can create a Filter Bubble