Left: CNN International. Right: CNN Domestic source 
Even before personalization news catered to existing biases. Compare the CNN International headline to the domestic one. Is it a protest or is it violence?
But newer algorithms and online behaviors allow for a somewhat disturbingly granular personalization of news. White Rabbit Holes allow extremism to seem moderate. Facebook's News Feed creates the Friendly Web in an effort to keep you happy and loose with your money. The Big Sort that happened in neighborhoods over decades happens in cyberspace over months.
In Twitter, Streams Don't Merge, with interesting political ramifications.