(CNN) -- Penny Ireland's family is so scattered around the world that Facebook, the popular social networking site, has become the family's No. 1 way to communicate.

The fastest-growing age group on Facebook is women older than 55, Inside Facebook says.

"We call it our living room," the 56-year-old mother said by phone from her home in Houston, Texas. "Everybody can tell what everybody else is doing."

"Everybody" includes Ireland's five kids and her 83-year-old mother, who has a Facebook profile she accesses daily, Ireland said.

While online social networks like Facebook, Twitter and MySpace are known hang-outs for younger adults and teenagers, older generations in recent months have been taking to the medium at a faster rate than any other age group, according to industry reports.

Many of these older folks use social networks to keep tabs on younger family members and they often find fruitful connections with their peers after they've friended all of their kids and grandkids, according to an informal survey by Stanford University professor BJ Fogg. Join a conversation on this topic at CNN's Facebook page

The trend is still relatively confined. Only about 7 percent of people older than 65 have online social-networking profiles, according to research from the Pew Internet & American Life Project.

But Facebook's popularity is growing most quickly among women older than 55, according to a site called Inside Facebook, which tracks Facebook's growth.

There are now about 1.5 million female users older than 55 on the site, the group says -- roughly a 550 percent increase over six months ago. By comparison, membership among people younger than 25 grew by less than 20 percent over the same period, Inside Facebook says.

Facebook now says it has 200 million users, making its user base larger than the populations of all the world's countries except China, India, the United States and Indonesia. Such a vast presence, coupled with news media buzz about all social media, has pushed online social networking to a "tipping point," said Amanda Lenhart, a senior research specialist at the Pew Internet & American Life Project.

Fogg, the Stanford instructor, said the trend has spread outward from college towns, where Facebook was first adopted, and inward from the American coasts. Facebook today has a global presence, with 70 percent of users living outside the U.S., the site says.

"We've reached critical mass where there's been enough talk about Facebook and people have gotten so many invitations from their friends, they're going, 'OK, what is this Facebook thing? I've got to get onboard or I'm going to be left in the dust,' " Fogg said.

Parents who are interested in their kids' online activities contribute to the sharp increase in older users on Facebook, said Linda Fogg-Phillips, a 49-year-old who, with her brother, co-taught a six-week class at Stanford called "Facebook for Parents."

"Parents are finally at the point where they realize this is not going away. They better figure out how to get on it and they'd better figure out how to use it," said Fogg-Phillips, who is a mother of eight in Las Vegas, Nevada. "It's a snowball effect. It's viral in a good sense."

Older people often must overcome fears about privacy issues before they will join Facebook, Fogg-Phillips said. Once they do, they often find unexpected uses for the network, she said. iReport.com: Facebook, Twitter growing pains?

That was the case for Craig Costa, a 55-year-old fly-fishing guide in Park City, Utah, who said family members forced him to join Facebook.

Costa still finds parts of the site annoying, and isn't comfortable having his personal information made public, but he has connected with old friends he wouldn't have otherwise -- including his ex-wife, who now is a Facebook friend of his current wife.

"It's been really interesting because so
What does one mother say Facebook has become? Who said Facebook was like her family's living room? What do some older family members use the site for? What is the fastest-growing age group on Facebook?
the family's No. 1 way to communicate. Ireland's to keep tabs on younger women older than 55,
question: What does one mother say Facebook has become?, answer: the family's No. 1 way to communicate. | question: Who said Facebook was like her family's living room?, answer: Ireland's | question: What do some older family members use the site for?, answer: to keep tabs on younger | question: What is the fastest-growing age group on Facebook?, answer: women older than 55,