What is it about ridiculously popular Web sites like Twitter, Facebook and YouTube that makes them so popular? In a word, community. That is, a loyal base of fans that use and contribute to the site.

Search engine provider Yahoo has embraced this notion as a way to rejuvenate itself as the company waits for regulators to, it hopes, sign off on a deal with Microsoft that would put an end to Yahoo's own search technology and replace it with Microsoft Bing. Since Yahoo is expected to retain control of the look of its search and portal pages, the company is looking to cultivate a community of application developers that can create new features and services to offer from its homepage.

Such community building was the main goal of Yahoo's ninth Open Hack Day, which kicked off today in New York City. During the event, Yahoo is introducing software writers to the company's user interface (YUI), database (YQL) and other developer tools in hopes of injecting more life into its Web sites. To sweeten the pot, Open Hack Day includes a 24-hour "hack contest" where programmers are encouraged to try out Yahoo's tools to build any type of application they choose, with the finished applications being presented and judged on Saturday.

Online communities thrive out of passion for what the community represents, whether it's broadcasting one's own diary (which is how blogs began), the ability to send short messages out to friends (Twitter's initial purpose) or the hunt for "things" (think eBay and Craigslist). A lesser-known example is MOCpages.com, an online forum for sharing ideas, information and accomplishments about LEGO creations.

History has shown that community sites grow when their destiny is placed in the hands of those using the services offered by the sites, rather than focusing on fancy technology that restricts users. For a site to grow, it has to map to the culture of its users, Clay Shirky, a new media adjunct professor in New York University's Interactive Telecommunications Program, said during his keynote Friday.

Shirky gave examples of community-building approaches that were instrumental to projects's successes, and of others that helped sink once-promising ideas.

The success of the Linux operating system is an example of a technology whose army of dedicated programmers and users propelled it from humble beginnings (creator Linus Torvalds wrote the operating system's core code in 1991 from his mother's Helsinki, Finland, apartment while he was still a student at the University of Helsinki) to become an alternative to Microsoft Windows and Unix that the even large corporations have embraced. The community behind Linux began with a simple post from Torvalds to the Usenet network that stated, "I'm doing a (free) operating system (just a hobby, won't be big and professional like gnu)." (GNU is an operating system similar to Unix.)

Torvalds miscalculation of Linux's potential in many ways contributed to its success because it drew computer programmers passionate about developing an alternative operating system based, to a large degree, on their input. By the time Linux's credibility was established to the rest of the world as an operating system to reliably run a wide spectrum of electronics, it already had a strong culture behind it, Shirky said.

Shirky cited the social-networking site Friendster—which ruled the scene briefly before the arrivals of MySpace, LinkedIn and Facebook—as a prime example of how micromanagement can kill a community. The beginning of the end for Friendster came in 2003 when it refused to let its users create fake Friendster profiles and began deleting profiles the company believed to be fake, according to Shirky. This imposition of rules put off a lot of Friendster users, leading them to search out more open social networks, according to TechDirt.com.

Image of Clay Shirky at Yahoo Open Hack Day ©ScientificAmerican.com/Larry Greenemeier