Before 2009, Facebook had given users a simple timeline��a never-ending stream of content generated by their friends and connections, with the newest posts at the top and the oldest ones at the bottom. This was often overwhelming in its volume, but it was an accurate reflection of what others were posting. That began to change in 2009, when Facebook offered users a way to publicly �like� posts with the click of a button.
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Shortly after its �Like� button began to produce data about what best �engaged� its users, Facebook developed algorithms to bring each user the content most likely to generate a �like� or some other interaction, eventually including the �share� as well. Later research showed that posts that trigger emotions��especially anger at out-groups��are the most likely to be shared.

First, the dart guns of social media give more power to trolls and provocateurs while silencing good citizens. Research by the political scientists Alexander Bor and Michael Bang Petersen found that a small subset of people on social-media platforms are highly concerned with gaining status and are willing to use aggression to do so.