Those ages 18 to 25 still make up the biggest proportion of users, at 35%. But people ages 26-44 now account for 41% of the Facebook audience. Women over 55 remain the fastest-growing demographic in the last three months, hitting 1.5 million.

Does this mean that ads for the Clapper and denture cream will soon proliferate on Facebook? Maybe not. But "developers and marketers want to think about how to serve this group of new users," wrote Inside Facebook founder Justin Smith.

Indeed, Facebook appears to be catering more to families as all ages become drawn into the social network. Another Facebook blog -- AllFacebook -- points out that the company has created a new landing page for creating private groups specifically targeted to their family.

It also notes the particular trend of moms going on Facebook. "I can't tell you how many friends of mine have complained about their mothers registering in the past couple weeks," wrote AllFacebook's Nick O'Neill. He acknowledges that "younger individuals" may not find that so cool. With its user base extending to parents as well as children, however, brands are taking note.

"In recent months, marketers have started to explore more opportunities to engage new audiences via Facebook beyond its traditional base of teens and college students, though the site still offers a lot of value in terms of that audience," said Art Sindlinger, vice president and social activation director at Starcom USA.

Adam Kasper, senior vice president and director of digital media at Havas Digital's Media Contacts unit, emphasized that many marketers are still getting their feet wet on Facebook. "While the growth of Facebook reflects an important demographic shift and it is now an established cultural phenomenon, placing ads on Facebook is still far from a must-have for advertisers," he said.

"More interesting for many of our clients is user activity on Facebook -- looking at how messages travel generationally from one person to another and the quantitative of the value of that pass-along."

While the demographic data highlighted by Inside Facebook focuses on U.S. users, its fastest growth has been internationally, with 70% of its more than 175 million active members in other countries. To keep up with its torrid expansion, the company is seeking to raise another $100 million in debt financing to lease more servers, according to a BusinessWeek report Thursday.

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