It happened the other day with little warning: Holly Braithwaite's mother asked her about Facebook, the juggernaut of Internet social networking sites.

Can you hear the sound of worlds colliding?

While Braithwaite knows she is old enough it shouldn't matter if her mom is on Facebook, the question still raised her defenses.

"Why do you want to know about Facebook?" Braithwaite asked her mother, who's in her 60s.

A friend had sent her an e-mail invite, her mom said.

"Just delete that," Braithwaite said, telling her mother Facebook would be too "difficult" and "complicated" for her to use.

"I don't know, it just seems so weird having my mom on Facebook," Braithwaite said in a recent interview as she explained her reaction to her mom.

"On one hand, I don't care. She can do whatever she wants. But on the other hand I'm like, 'Man, that's my mom, and that's weird.' "

But like it or not, an older demographic is joining Facebook. The site was originally started as a networking site for college students, but three years ago it was opened to people of all ages.

Now, there are more than 175 million active users on Facebook and greater than half are outside college. While Facebook says its fastest-growing demographic is people over age 30, the blog "Inside Facebook" believes women over age 55 are one of the top groups joining the site.

Either way, consider this: During the past four years, the percentage of adults on social networking sites has more than quadrupled from 8 percent to 35 percent, according to the Pew Research Center. Of people ages 35-44, at least 30 percent have a profile on a social network.

Jump 10 years to people ages 45 to 54 and 19 percent have profiles on social networks. Another 10 percent of people ages 55 to 64 have profiles.

Marlene Hyte, of Cottonwood Heights, said she joined Facebook a few months ago after hearing about it from her daughter. She's in her 60s and uses it to stay in touch with family, friends and also long-ago acquaintances.

Updates are sent to her BlackBerry, said Hyte, who considers herself very tech-savvy.

"I have instant access to what's going on," said Hyte. "My grandchildren are on there. My nieces and nephews are on there. When they post something, I have an instant update."

Her husband, Ron Hyte, is also on Facebook and said he and his wife have a competition to see who gets "friended" first by a person. With Facebook, people create profiles and only people they have invited as "friends" can see their profile.