Anyone who works for a major news website or publisher knows that social referrals—that is, links that are shared on social networks such as Facebook and Twitter—have become a crucial source of incoming traffic, and have been vying with search as a source of new readers for some time. Now, according to new numbers from the traffic-analytics service Parse.ly, Facebook is no longer just vying with Google but has overtaken it by a significant amount.
Parse.ly’s chief technical officer Andrew Montalenti said in an interview with Fortune that the company’s latest estimates show that social-media sources (of which Facebook (FB) is by far the largest) accounted for about 43% of the traffic to the Parse.ly network of media sites, while Google accounted for just 38%.
The company’s clients include more than 400 major news and media outlets, including traditional publishers such as Wired, The Atlantic, Reuters and The Daily Telegraph, as well as a large group of digital-only outlets such as Mashable, The Next Web, and Business Insider. Collectively, the network accounts for about 6 billion pageviews and more than one billion unique visitors per month.
This isn’t the first time that Facebook has edged past Google in the traffic-referral race, Montalenti said. The social network (which is No. 10 on Fortune’s 2015 Fastest Growing Companies list) took the top spot by a small amount last October, but this month’s lead is far more dramatic—and the Parse.ly CTO said that from the company’s data, it’s clear that search has hit a kind of plateau and isn’t really growing any more as a referral source for media. Meanwhile, Facebook’s influence has “shown it’s on a continued growth trajectory.”
That trajectory has been fairly dramatic: According to Parse.ly, as recently as January of last year, Facebook accounted for just 20% of all the traffic from documented sources to the company’s network of media sites, and now it is more than double that. Montalenti said this doesn’t mean Facebook accounts for 43% of all traffic, but just the sources that Parse.ly is able to get data on.
Tony Haile, CEO of the traffic-analytics firm Chartbeat, said that his company’s data shows a similar theme: although Google has a much broader range of sites it sends traffic to via search, the larger news and media sites have become much more reliant on Facebook. “When we look at all the sites over our network, a third have more Facebook traffic than Google,” he said. “But when we only look at the largest 20%, about half of them have more traffic from Facebook than Google.”
Although there’s no question that social sharing has become a much more important source of traffic in its own right, the Parse.ly CTO said that a change in Google’s referral practices may also have played a role. While the search company used to show publishers what keywords were used to direct the most traffic to their articles, in most cases it doesn’t provide that kind of data any more. The company says this is for security reasons, but Montalenti says it could also be because Google doesn’t want publishers to game its algorithm.
Whatever the reasoning, that lack of insight into what kind of traffic is coming from Google and why could have helped contribute to a lack of interest in SEO and growing interest in using social platforms such as Facebook and Twitter (TWTR). Facebook has also been trying to court media companies and get them to host more of their content on the site, Montalenti noted, through efforts such as the Instant Articles project it announced earlier this year.
The only problem with that shift, the Parse.ly CTO says, is that Facebook is almost as impenetrable as Google when it comes to trying to figure out why one article did well and another didn’t. And that makes it difficult for publishers to build a coherent social-media strategy.
“There’s a lot of effort among media companies being placed on specific social channels like Twitter, but our data shows that Twitter is basically a distant traffic source,” says Montalenti. “That’s unfortunate because Facebook is a lot less transparent around things like how the algorithm functions. There’s a lot more useful data from Twitter about their content, but FB is more like a black box in terms of how it operates. And yet it’s this huge and growing traffic source.”
You can follow Mathew Ingram on Twitter at @mathewi, and read all of his posts here or via his RSS feed. And please subscribe to Data Sheet, Fortune’s daily newsletter on the business of technology.