Even Facebook itself sees the segmentation of users along political lines on its site - and synchronizes it not only with the posts users see, but with the advertisements they’re shown.
Test it out yourself: Go to facebook.com/ads/preferences on your browser and click the “Lifestyle and Culture” tab under the “Interests” banner. You see the box titled “US Politics”? It’s followed with a parenthetical notation of your political alignment, from “Very Conservative” to “Very Liberal”.
Platforms – and traffic-hungry websites – have followed the behavioral lead of Facebook’s users. “News sources” – largely aggregators of video clips and interviews from other sites – that barely exist beyond the sharing economy of Facebook have arisen as major players in the site’s political news sphere.