Ask just about any college student you know, and he is likely to tell you that he is a member of facebook.com. Many students say they check facebook more often than they check their e-mail. It's so popular "facebooking" has become a verb.
"Everywhere everyone is doing it," college sophomore Katie Silverman said. "It's like everyone's center of life."
Facebook is an online social forum where students create profiles, share personal information, and meet other members. It is password protected and targeted to high school and college students; members must have an .edu e-mail address to join.
Many students say it has revolutionized their social lives, giving them an easy way to find and chat with classmates who share their interests -- and flirt.
"Some people might be too shy to go talk to someone they like in one of their classes, but on facebook it's a lot easier to strike up a conversation," Rahul Vanjani said.
The Web site was founded by two Harvard University students who wanted to meet more of their classmates. It became so popular at Harvard that they expanded it to other campuses.
Now, according to facebook, the Web site is on 2,200 college campuses and at 22,000 high schools. It says that it has 6 million active users, two-thirds of whom log on every day.
Many of the facebook members that ABC News spoke to said their parents had no idea they were on it.
"No way. My folks definitely don't know about it," said one Washington, D.C., area college senior who asked us not to use his name.
While facebook says it bans nudity on the site, students regularly post other intimate and inappropriate information that could damage their reputations. In many instances, underage kids write about using drugs, getting drunk, and having sex. Some describe their wild nights or write critical comments about other classmates.
Some students have paid the price.
At Fisher College in Boston, two students were expelled for using facebook.com to threaten to commit a crime. At North Carolina State University, four students were disciplined for posts about drinking.
When asked by ABC News whether campus police or administrators should monitor the site, high school and college students said that would be a violation of their privacy. But administrators don't necessarily see it the same way. Some schools have banned facebook from their campus computers. And security experts are warning kids and parents to beware of what they post.
"They need to think before they click," said Parry Aftab, the executive director of wiredsafety.org. "They shouldn't post anything that their parents, principal or a predator shouldn't see. And they should password protect everything they can."
She also says students' online comments can hurt them when they apply to college -- because many admissions officers can access the Web site, read their posts, and make judgments about their character. Several employers admit to reviewing applicants' online profiles before making hiring decisions.
"Mary," a high school junior in Virginia, says she doesn't believe her friends are thinking about college counselors, future employers, or even their parents when they post on the site. She even laughed at the question.
"That's kind of stretching it. When you're our age and writing something, I don't really think it's going to come up again, like, 10 years from now."
But some school administrators say that's exactly what worries them.