User-generated content is essential for web success, and new research from the Duke University Fuqua School of Business shows how much content sites need to generate in order to attract active users.

"Sites need critical mass," Professor Carl Mela said. "But it's a chicken and egg problem: If there are no readers there is no reason to post, if there are no posts there is nothing to read."

Several of the most visited sites on the web — including YouTube, Wikipedia, Ask, Amazon and its Chinese cousin Taobao, operated by Alibaba — are fueled by user generated content in the form of reviews, posts and comments.

The challenge sites face is how to jump-start online communities that become self-sustaining, posting and consuming content, without the site doing all the work.

Mela, working with Dae-Yong Ahn of Chung-Ang University in Seoul, South Korea, and Jason Duan of the University of Texas, developed a model for how much content a social site needs to create or sponsor in order to tip a network into self-sustainment.

Their findings, "Managing User Generated Content: A Dynamic Rational Expectations Equilibrium Approach," are forthcoming in the journal Marketing Science.

The research team studied two months of user log files from an online forum — not identified because it shared proprietary data — with more than 150,000 registered users. The files showed how many posts each registrant visited, read and posted, and when.

"We had the data to observe when people engage with content, unlike when people read newspapers or talk in the hallway," Mela said.

They found that content created by the site, or by paid users, is the most effective way to tip a network into self-sustainment.

"Our model actually predicts an optimal level of site generated content, above which the site begins to hurt itself in terms of overall traffic," Mela said. "If the site generates too much of its own content, it begins to compete with users' contributions for reader attention. This lowers the incentive for users to contribute."

In the researchers' model, the site needs to be creating or generating between 5 percent and 10 percent of its content. If the content — reviews, videos and so on — is of sufficiently high quality, that proportion can be driven as low as 1 percent.

"On a mature site, if you're going to jump start it using high quality site content, you need create only about 1 percent of the content once the site is well established," Mela said.

Lurkers — visitors who read regularly but rarely post, and who make up the majority of visitors to most sites — also play an important role in site development, the researchers found. Reducing the number of lurkers results in sites needing more user content needed to make a site self-sustaining.

"Lurkers help draw content generators in," Mela said. "I'm more likely to post if I believe what I post is more likely to be read."

The research can help companies launching online forums calculate how much to spend on developing content, where to focus their efforts — and how much is too much.

"If a site throws out a lot of compelling content, readers will come," Mela said. "But if the site posts too much material, then it's in competition with what users post. Early on, site-posted content works well to drive people to the site, but as more and more people engage with the site and participate, the site should ease back on its own content to stop competing with its users."