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Google used to be the main source of referral traffic for web publishers. Then Facebook eclipsed it.

And now, Google is back on top again.

Over the course of 2017, the search engine has become publishers’ main source of external page views, according to new data from Parse.ly, a digital analytics company.

It’s basically a flip from the beginning of the year: In January, Facebook provided nearly 40 percent of publishers’ external traffic; now that’s down to 26 percent. And Google, which started the year at 34 percent, now generates 44 percent of traffic. Parse.ly’s data comes from some 2,500 publishers that use its analytics service, including the Wall Street Journal, Time Inc., Mashable and Huffington Post.

What happened?

You can point to a number of factors, but there are a few obvious ones to consider:

Facebook is constantly updating its news feed algorithm and any changes to the way Facebook surfaces stories could have far-reaching effects on publishers. Last year, for example, the social media company tweaked its algorithm to prioritize posts from friends and family over publishers.

Facebook’s “Instant Articles” feature, where the service hosted some publishers’ content directly but promised to send more readers to the original site as well, has declined in importance.

There’s been a broad move toward publishing video directly on Facebook, which could affect how many links to web stories publishers put on their Facebook pages. Any algorithms on Facebook that prioritize native video over text links could have an effect, too.

Google’s AMP — accelerated mobile pages — feature, which also hosts publishers’ content directly on Google’s servers, has become more important. AMP stories — typically from news publishers — are surfaced at the top of mobile search results as “Top Stories,” which drives clicks.

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