Facebook?s younger members - high school or college students, and recent graduates who came of age as Facebook got its start on campuses - appear comfortable with sharing just about anything. It?s the older members - those who could join only after it opened membership in 2006 to workplace networks, then to anyone - who are adjusting to a new value system that prizes self-expression over reticence.

Facebook says it is the world?s largest social network, with 175 million members. But in the United States, most members are still relatively young; Facebook offers advertisers a target of 54.4 million members of all ages. But if an advertiser wants to narrow its target audience to those 25 or older, the number drops to 28.8 million. Narrow it to those 30 or older, and Facebook has 20.3 million to offer.

Many over-30 graybeards have yet to sign up, so Facebook has a chance for astonishing growth. Each week, a million new members are added in the United States and five million globally; the 30-and-older group is its fastest-growing demographic.

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