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.