Many of the people who read this article will do so because Greg Marra, 26, a Facebook engineer, calculated that it was the kind of thing they might enjoy.

Mr. Marra’s team designs the code that drives Facebook’s News Feed – the stream of updates, photographs, videos and stories that users see. He is also fast becoming one of the most influential people in the news business.

Facebook now has a fifth of the world – about 1.3 billion people – logging on at least monthly. It drives up to 20 percent of traffic to news sites, according to figures from the analytics company SimpleReach. On mobile devices, the fastest-growing source of readers, the percentage is even higher, SimpleReach says, and continues to increase.

The social media company is increasingly becoming to the news business what Amazon is to book publishing – a behemoth that provides access to hundreds of millions of consumers and wields enormous power. About 30 percent of adults in the United States get their news on Facebook, according to a study from the Pew Research Center. The fortunes of a news site, in short, can rise or fall depending on how it performs in Facebook’s News Feed.

Though other services, like Twitter and Google News, can also exert a large influence, Facebook is at the forefront of a fundamental change in how people consume journalism. Most readers now come to it not through the print editions of newspapers and magazines or their home pages online, but through social media and search engines driven by an algorithm, a mathematical formula that predicts what users might want to read.

It is a world of fragments, filtered by code and delivered on demand. For news organizations, said Cory Haik, senior editor for digital news at The Washington Post, the shift represents “the great unbundling” of journalism. Just as the music industry has moved largely from selling albums to songs bought instantly online, publishers are increasingly reaching readers through individual pieces rather than complete editions of newspapers or magazines. A publication’s home page, said Edward Kim, a co-founder of SimpleReach, will soon be important more as an advertisement of its brand than as a destination for readers.

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