Social bookmarking sites such as Digg and reddit drive traffic to newspaper web sites. It's a proven fact. Except when it isn't.

That's the frustrating conclusion of an article published by the Newspaper Association of America as part of the organization's series on social media ROI. Paul Berger writes:

Today, readers can choose from dozens of bookmarking sites - StumbleUpon, Newsvine, Mixx, Propeller, Twine, Diigo, Blinklist, Blogmarks, Yahoo! Buzz - not to mention social networking sites such as Bebo, MySpace, Facebook and Twitter. But what is striking is the different experiences newspaper editors have had trying to leverage such sites for traffic or brand-building purposes.

The ChicagoTribune.com, for example, reports having solid success with Digg, which refers about 5% of the site's total traffic, about half the total delivered by social media sites overall.

Then there's Minneapolis-St. Paul's StarTribune.com, which, Berger quotes the newspaper's digital executive director Jason Erdahl, gets "less than one-hundredth of a percent of overall visits" from the Diggs, reddits and Newsvines of the world.

Berger reports:

Erdahl says Drudge, the Huffington Post, and particularly Yahoo!, are far more significant referrers to StarTribune.com than social bookmarking sites. Facebook and Twitter are also important, he says, because they allow people who already know and trust each other to make recommendations.

In fact, Berger writes, Facebook and Twitter are emerging as far more effective traffic-drivers for local newspapers than social bookmarking sites.

The article offers some tips on social media strategies for newspapers, but the advice certainly can be generalized to online-only publishers.

My bottom line: There's no magic formula for social media. You've got to find what works for your publication. But that means trying things, taking chances and paying attention. Newspapers should adapt the advice of Richard Pryor, who explained his philosophy of comedy thusly: "Whatever the fuck make the people laugh, say that shit."