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When Amy Waldman first signed on to Facebook last year and started to send joking messages about good grammar back and forth with a new 18-year-old friend, Waldman’s 19-year-old daughter, Talia, upbraided her for not revealing that she was actually in her 40s.

“You have to tell her you’re old,” she explained, “because on Facebook, that’s creepy.”

Waldman created a Facebook group to commemorate the incident: “over 40 is ‘facebook creepy.’ “

It’s no secret that Facebook, which started as a networking playground for college kids, is graying, and that the percentage of active members who are over 25 years old and out of school has risen to some 40 percent of the overall population of more than 43 million.

The influx raises questions. Will the loss of the campus sensibility and the youthful vibe dilute the Facebook experience? And will the newcomers use the site — and change it? Joe Uppal, a University of Michigan undergraduate studying anthropology and philosophy, said the age wave “does kind of undermine the ties between Facebook and the college community.

“If everyone and anyone is able to go on Facebook, then belonging to it no longer indicates a college-student identity,” he added.

Similar sentiments have been heard whenever Facebook has expanded, said Matt Cohler, the company’s vice president for strategy and business operations. When the fledgling company first expanded beyond its roots at Harvard to include Yale, he said, “The people at Harvard complained about it and said, ‘Hey, this used to be just for us!’ ” The fretting subsides each time, he said, “as soon as they go back to using the program and realize this hasn’t done anything to deteriorate their experience.”

For the most part, in fact, the entry of millions of older people has been greeted with an epic “whatever,” said Clay Shirky, an adjunct professor in New York University’s interactive telecommunications program. “The average Facebook user isn’t going to care that people utterly unlike them are doing things they utterly don’t care about on some other corner of the site,” he said.

Facebook can avoid its “there goes the neighborhood moment,” he said, as long as it allows people to stay in their silos and gives them control over who can peek at their profiles. “As long as the individual users still feel their culture is preserved in their corner of Facebook, the growth won’t bother them.”

Waldman — she of the creepy 40s — finds herself using Facebook in many of the ways that younger people do, except for the sexual cruising: She keeps up with old friends, makes new ones and uses the network to, well, network.

For instance, it turned out that Waldman’s age, 48, was just fine with the young woman, Lisa Szczepanski, with whom she traded grammar jokes. Szczepanski lives in Queens, and Waldman lives in Milwaukee, but they have become as close as electrons allow. Waldman calls Szczepanski “My Facebook kid” and got a Mother’s Day card from her last spring. (“Even my own kids don’t do that!” she joked.)

Young and old will inevitably use the technology of Facebook differently, said Nicole Ellison, an assistant professor in the telecommunications, information studies and media department at Michigan State University.

Students define themselves through membership in many social networks in the real and virtual worlds, including classroom, dorms, extracurricular activities and hobbies. “There are fewer of those for adults,” Ellison said.

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Facebook.com

More than 43 million users, three times the number of users in January, according to Malorie Lucich, FaceBook spokeswoman

Target user: Social networking for college students, high school students and recent alum. Previously limited to college students, now open registration for anyone, including businesses.

Pros: Reconnecting with friends from school eliminates the need to attend; easy to navigate and post event notices; user-friendly photo-sharing; specified motives, like dating, friendship, random play.

Cons: Your boss wanders across photos of you taking shots of whiskey with underpants on your head; layout can’t be totally customized; news feeds track recent activity and updates subscribers with every move, such as if relationship status changed or if your ex wrote on your wall.

What’s new? Users can add to their profiles applications including daily horoscopes, movie trivia quizzes, slideshows and games; instead of the old method where users could ‘poke’ one each to interact (like a digital guest book), new applications allow users to make their presence known in an uncommon way such as “throw a sheep” or “send a cocktail.”