Facebook now has more than 400 million active users, up from only 50 million as recently as 2007. If social networking still resembled a young, hip downtown nightclub scene — one day a site is hot, the next it’s not — we might expect the crowds to decamp soon. Facebook would become another Friendster, still around but ghostly, forgotten by most.

Facebook, however, isn’t likely to have such a fate. For one thing, it has attracted many “olds,” and they tend to stay put. (Consider AOL.) More than 50 percent of Facebook’s members in the United States are 35 or older, and only 26.8 percent are 24 or under, according to an analysis of December visitors by comScore Media Metrix.

More than demographic stability favors Facebook. The site has shrewdly emulated the “network effects” strategy used by another brand that has long held a dominant position in the computer industry: Microsoft Windows.

Economists use the term network effects to refer to the way the value of a product or service increases in tandem with the number of people who use it. If you’re one of only 10 people in the world with an e-mail account, its usefulness is limited; add a billion more, and the practical value of yours increases apace.

A Facebook member enjoys immediate benefits when each friend joins — these are direct network effects. But the average user already has 130 friends, so unless the user is unusually gregarious, the direct effects won’t increase drastically beyond a certain point.

For an individual member, the most powerful network effects may be indirect ones that come from the huge number of unknown other people in the Facebook world. Their mass attracts, in turn, suppliers of complementary products and services.

For Windows, the enormous installed base attracted third-party software developers, which in turn drew more users. Apple’s iPhone has had a similar virtuous cycle. So, too, on Facebook, developers of applications like FamilyLink, Marketplace and iLike’s Music create a software universe with seemingly infinite choices. And that attracts more users — and still more developers.

Facebook’s decision to open its site to outside developers in May 2007 was a “transformative moment,” said Charlene Li,founder of the Altimeter Group, a strategy consulting firm.

“Because Facebook allows developers on their site, the people who would have developed the next social networking site are now working with Facebook,” she said.

Nick O’Neill, founder of AllFacebook.com, a site with Facebook-related news and statistics, said, “Games are the killer app for Facebook.” Because of their social nature, popular Facebook games produce direct network effects. The dedicated farmers of the FarmVille game — it attracts 83 million users a month — nudge friends to play and become virtual neighbors, enhancing their own game experience. (That pull gives Facebook an advantage Windows lacked; its signature game was Solitaire.)

Businesses, nonprofits, government offices and celebrities use Facebook pages to disseminate information, thus forming an ever-growing simulacrum of the Web within Facebook’s walls. (NYT)