The Chilmark Police Department recently shut down a longtime party barn for teens after a source led them to photos of the location on Facebook. The photos were posted on the social networking Web site by the very kids who took part in the booze-infused parties held there, begging the question: Why post a public record of illegal or unsavory behavior in the first place?

It’s not mere recklessness or indifference. Teens just seem not to comprehend how public their private Web pages can be. But Island law enforcement officials, school officials and parents are working hard to warn kids to be more careful.

“We tell them: don’t assume that it’s private information you’re posting there,” said Martha’s Vineyard Regional High School guidance director Michael McCarthy, “It becomes very public, very fast.”

Mr. McCarthy said he has seen social networking-related issues grow steadily since the introduction of MySpace and Facebook; particularly during the past year or so, as Facebook grown to include users of all ages and demographics.

As it turns out, losing a favorite hangout is the least of these kids’ worries. Facebook has become a preferred forum for informal background checks by college admissions counselors, potential employers, and law enforcement officials. A decade ago, a snapshot of an underage kid with a beer in his hand at a party was most likely hidden away in a drawer away from parents. With the advent of Facebook, kids are posting photos of drinking, drug use and other illegal activities on the Internet under the illusion of privacy.

“It’s not a private photo album, it’s open to the world,” said Chilmark police officer Sean Slavin in an interview at the police station last week. “I think they think they can insulate themselves by blocking certain things on Facebook,” he said, referring to a Facebook privacy tool that allows kids to choose who can and cannot see certain photos and messages. “What they don’t realize is that there are ways that that just doesn’t work. Those pictures can come out, and not by any special law enforcement tool. They’re not as secret as [the kids] think they are,” he said.

Chilmark police chief Brian Cioffi stood across from officer Slavin, holding a stack of papers an inch thick, all photos of underage drinking parties and transcripts of internet “chats” between Island high school students.

“And you can print,” said chief Cioffi. “Here are 56 photographs from Facebook . . . . So now there’s a hard copy.”

Chief Cioffi said a concerned citizen — not a student involved or one of their parents — led the Chilmark police to the photos on Facebook that helped in the investigation of the underage drinking spot. Which proves that anyone can get access to the material.

“What you put on the internet is in the public domain now,” said Chief Cioffi.

Vineyard schools superintendent Dr. James H. Weiss said that photos and information posted on Facebook are becoming more likely to haunt kids later in life. “Many large companies use these social networking sites as part of their recruitment process,” he said. “They’ll use Monster.com to get people to submit applications, and then they will go on their Facebook pages. If they see things that are totally unacceptable, even if they’re from five, six years ago . . . that can clearly work against somebody getting hired.”

Dubious Facebook posts can even threaten their chances of getting into college. Mr. McCarthy said that the topic came up at a recent college board conference he attended. “A number of admissions counselors said, ‘We’ve gone in and friend [requested] kids, and looked at their Facebook accounts to see what we can see,’” he said.

“Many colleges now, as part of their enrollment process and recruiting process and admission process will go on Facebook and all of the other social networking sites and see what people have put up there,” said Dr. Weiss. “That could work negatively against your admission.”

Dr. Weiss said the school has taken measures to inform people about the dangers of Facebook. At parent workshops, high school and middle school parents have been encouraged to educate themselves about the internet sites and to monitor their children’s use.

“Parents need to become tech savvy,” he said. “You need to have that discussion with your kid, and you need to have it on an ongoing basis, like you would with anything else. That’s one of those discussions that parents have an obligation to have with their kids: You can do yourself irreparable harm over the long term if you do this.”

Mr. McCarthy said that he has seen many parents take steps towards monitoring Facebook use, including joining the site themselves. “More and more parents are on [now],” he said. “So I do feel like adults are catching up with the technology a little bit.”

Other programs for teenagers are getting involved as well. The Youth Task Force, which strives to reduce underage drinking, drug use and other dangerous behavior for teens, plans to introduce an Internet safety link on their new Web site, due out next month.

Task force coordinator Theresa Manning said, “We have a youth group, and we’ve recently been having some dialogues with the police departments around Facebook, and just the kids’ inability to filter what they’re putting up. We’re beginning to talk about it and try to figure out some strategies to help kids make better choices.”

But teenagers often have difficulty measuring the impact of their decisions. “Their brain, their impulse control, everything is not really developed yet, so at their age they’re still drawn to do the things that feel better than restraining themselves,” said Ms. Manning.

The teenagers in question aren’t bad kids, said Chief Cioffi and officer Slavin. Instead, they are average kids engaging in behaviors not uncommon for their age. The problem is that they are allowing Facebook to give just about anyone access to records of this behavior. And what might seem now like a fun social outlet could have long-term consequences for their education or career.

“We’ve always been told that whatever you do on a computer is somewhere. You can’t get rid of it,” even by deleting it, said Chief Cioffi. “It may not be on your page anymore, but there’s a record of it somewhere.”