User participation is crucial to the survival of popular websites like YouTube and Flickr. But how do these sites ensure that new videos, photos and comments keep flooding in?

It all comes down to persuasion strategies, says B. J. Fogg at Stanford University in California, who is analysing the techniques employed by websites that rely on their users for content, known collectively as Web 2.0. The secret is to tie the acquisition of friends, compliments and status - spoils that humans will work hard for - to activities that enhance the site, such as inviting new users and contributing photos, he says. "You offer someone a context for gaining status, and they are going to work for that status."

Fogg and his colleagues analysed hundreds of such sites and identified three stages to their success, which they called discovery, superficial involvement and true commitment.

They found that the first two ...