The future of news may be online, but even the biggest of the big news outlets have a little problem on that front.Turns out most people don’t come to their sites very often, and when they do, they don’t stay very long.Most are “casual users” who stop by only a few times a month; 85 percent of visitors to USAToday.com, for instance, came between one and three times a month. And once there, 30 to 40 percent of visitors to top news sites stay less than five minutes.The figures are from a Project for Excellence in Journalism study of online news habits, out Monday, that is certain to be closely scrutinized by media types. I can’t tell you how much energy is expended in newsrooms trying to figure out how to boost traffic and get readers to stick around.The project looked at 25 top news sites, including The New York Times, L.A. Times, and Washington Post, Huffington Post, Yahoo, AOL, CNN, MSNBC, Fox and London’s Telegraph and Guardian.These sites have a core of loyal readers—about 7 percent—dubbed “power users,” who return more than 10 times a month. The figure runs as high as 18 percent at CNN.com. For The New York Times, 9 percent are heavy users—the target audience for its new paywall.The findings on who drives traffic to these news operations are striking. Google is the monster, accounting for 30 percent of the readers at the top news sites. One site, Examiner.com, got just over half its traffic from Google. The Drudge Report remains quite influential, steering 19 percent of traffic to the New York Post and 15 percent to The Washington Post, for instance.But the rise of social networking may steal the headlines. Facebook is becoming a major player. With 500 million users worldwide, Mark Zuckerberg’s creation was the second- or third-most popular traffic driver for six of the top news sites studied. Arianna’s Huffington Post got 8 percent of its visits from Facebook links; The New York Times drew 6 percent. The proliferation of “like” buttons make it easier to follow such links.Twitter, with 175 million accounts, was a minor player, a traffic driver for just nine of the 21 news sites for which data are available. And for all but one of those, Twitter provided only 1 percent of the traffic. The exception: LATimes.com got 3.5 percent of its traffic through tweets.But for all the off-ramps entangling the Internet, the home page still rocks. It was the most viewed page for 21 of the 25 news sites, accounting for 79 percent of the hits at Reuters.com and 60 percent at CNN.com, although just 6 percent at AOLNews.com. In that sense, many of these sites still resemble newspapers: People want to check the day’s headlines.