Facebook is driving an increasing amount of traffic to news websites but Google is still the top referring service, according to a study published today.

The study by the Pew Research Center’s Project for Excellence in Journalism looked at the behaviour of news consumers online during the first nine months of 2010 using audience statistics from the Nielsen Co.

The study examined the 25 most popular news websites in the United States, looking at how users get to the sites, how long they stay there, how deep they explore a site and where they go when they leave.

An average of 40% of the traffic to the top 25 news websites arrives from outside referrals, the study found, with Google Search the single biggest driver, accounting for 30% of all traffic.

But social media – and Facebook in particular – is increasingly becoming a major driver of traffic to the top news websites.

On five of the top 25 websites, Facebook was the second or third biggest driver of traffic. The Facebook Open Graph API is clearly having an effect on global news sharing and dissemination, with the ‘like’ button now permeating cyberspace.

Twitter, surprisingly, “barely registers as a referring source,” according to the study.

Many visitors to top news sites are what the study described as “casual users” – people who visit just a few times a month, spending just a few minutes on the website with each visit.

This is perhaps evidence of the impact of information sharing across social networks such as Facebook. With people sharing news and features with their peers, this is likely to draw in a more ‘casual’ visitor – those with not a specific interest in the story or publication in question, but who are keen to know what their friends are reading about.

Indeed, on average, 77% of the traffic to the top 25 news sites came from users who visited just one or two times, the study said, with the percentage varying among sites.

More loyal and frequent visitors – what the study called “power users” – return more than 10 times per month to a particular site and spend over an hour there over that time.

Only six sites had “power user” figures in double digits.

“Overall, the findings suggest that there is not one group of news consumers online but several, each of which behaves differently,” the study said.

“These differences call for news organizations to develop separate strategies to serve and make money from each audience.Advertising may help monetize some groups, while subscriptions will work for others.”

The top 25 sites included 11 newspapers: The New York Times, The Washington Post, USA Today, The Wall Street Journal, The Los Angeles Times, the New York Daily News, the New York Post, the Boston Globe, the San Francisco Chronicle, the Chicago Tribune and Britain’s Daily Mail.

Six were the websites of broadcast or cable television networks – MSNBC, CNN, ABC, Fox, CBS and the BBC – and four were pure news aggregators – Google News, the Examiner, Topix and Bing News.

Three were “hybrid” sites which mix aggregation of other news sources with original reporting – Yahoo! News, AOL and The Huffington Post – and one was a wire service, Reuters.