Even though Slashdot’s traffic continues to grow, now ranging from 300,000 to 500,000 visitors a day, links on Slashdot no longer reap the overwhelming numbers of clickthroughs they used to. Chalk it up to the ever-expanding Web.

A couple of years ago, Omid Rahmat, general manager and publisher of TG Publishing’s TomsHardware.com, would practically do cartwheels every time a story from his site ended up on Slashdot.org. Attracting millions of techies from all over the world with its links to the most important tech news of the day, Slashdot would send Tom’s traffic up by 30%, providing the site with much-needed exposure.

This “Slashdot effect” has rippled through the tech news industry, where higher traffic numbers tend to translate into greater ad revenues. In the past several years, many editors encouraged reporters to pick story ideas that were likely to make it onto Slashdot. Some writers even submit their own stories to Slashdot in hopes of generating more traffic to their home page and earning kudos from their bosses.

Ethical considerations aside, such efforts don’t have the payoff they used to. Over the past year, the Slashdot effect has begun to fizzle. Nowadays, a mention on Slashdot typically increases Tom’s traffic by just 5% to 10%, Rahmat says. The boost also is more temporary, “usually peaking within one to two hours and almost completely over in three,” Joel Johnson, editor of gadgets site Gizmodo.com, writes in an e-mail to BusinessWeek Online. In fact, he writes, Gizmodo.com now gets as much traffic from other sites, like Fark.com, which collects offbeat news.

A year ago, Slashdot contributed 5.5% of all traffic going to tech news sites like CNET (CNET ), Wired, and Gizmodo. But by last month, its power had fallen considerably, estimates Bill Tancer, an analyst at online measurement company Hitwise in Redwood City, Calif. When Tancer measured traffic going from Slashdot to several recent stories to which it linked, he found that, in one case, Slashdot increased a site’s traffic by 4%. In the case of one Reuters wire-service story, no one clicked on the link at all.

That’s happening even though Slashdot’s traffic continues to grow, according to co-founder Rob Malda. At 300,000 to 500,000 visitors a day, it has six to seven times more visitors than it had four years ago.

How can this be? The number of news sites Slashdot is linking to has skyrocketed. And that has reduced the impact Slashdot can make on each individual site’s traffic. The number of tech news sites, run by traditional media companies, reaches 360 today, up 20% from 300 just one year ago, according to Hitwise. These sites have proliferated following a revival in U.S. online ad spending, which is projected to grow by more than 20% in 2005, to more than $11 billion, according to e-commerce consultancy eMarketer.

More here.