Data journalists pulled 26,234 of Trump's 34,062 tweets (dating from Jun 1 2015 to Nov 17 2016) from the Twitter API and analyzed them for news-sources, producing a long, detailed analysis complemented by interactive graphics.
It's an echo-chamber's echo-chamber. For example, Trump does tweet a surprising amount of "mainstream media" (sometimes repeatedly tweeting the same stories over the course of days), but only when they say complementary things about him. As to the "non-mainstream" sources that Trump favors, their overwhelming commonality is that they quote Trump a lot.
An interesting corollary to this is that to the extent that Trump himself is a broken machine-learning algorithm with a biased sample in his training data, he is also a source of training data for self-interested, possibly apolitical media sites that are figuring out that saying nice things about Trump gets them tweeted by the most powerful person on earth, which gets them a lot of ad-clicks.
Trump's most-tweeted source is, of course, Breitbart.
Trump's Twitter account is just one part of the president-elect's information diet, but it's an instructive one. With its broad reach comes considerable influence; a BuzzFeed News analysis found that Trump's average news tweet receives about 10,265 engagements (a combination of retweets and favorites) with a median engagement of 4,729, while his top news tweets garnered well above 53,000 total engagements.
Throughout the campaign, Trump's engagement from his account outperformed Hillary Clinton's substantially. In the three months leading up the election day (Aug. 9 to Nov. 8), Clinton's account tweeted 2,449 times with an average of 3,964 retweets; Trump tweeted 587 times with an average of 10,863 retweets. And many of Trump's biggest non-news tweets pulled in hundreds of thousands of total engagements. Thanks to Trump's facility with Twitter and his uncanny ability to use it to simultaneously bypass and program traditional media, the account has been a uniquely powerful megaphone for his candidacy — and an unconventional preface for his presidency to come.
Here's Where Donald Trump Gets His News [Charlie Warzel and Charlie Warzel/Buzzfeed]