(MONEY Magazine) – If you're the parent of a teen, you're probably hip to the dangers of social-networking websites like MySpace.com and Facebook.com. Identity thieves and sexual predators famously lurk among the millions of ingenuous kids. And chances are you've satisfied yourself that your kids are hip to these much discussed perils as well.

But here's one few people--parents or kids--have considered: that users may be unwittingly creating a kind of shadow résumé that will hurt their employment opportunities for decades to come.

"If you've been out there talking about yourself and posting photos for 10 years, it can have an impact on your job hunt," says Pam Dixon, who heads the World Privacy Forum. Human-resources departments already Google candidates routinely, she says, and "to say that it won't affect employment prospects is naive."

Between them, the sites had 37 million hits in November, most of them by young people. Browse a few random profiles and you'll see that MySpace (the more open community of the two) is filled with the kinds of unguarded banter and posturing that routinely go on when adults aren't around. Users share gossip, photos, favorite bands and, often, their innermost thoughts--everything from inchoate (but passionate) political views to contemplations of suicide to favorite sexual positions. The most popular verb seems to be "party!"

Employers could never ask about such things, but if it's on the Web, they're entitled to make decisions based on it. And as corporate America increasingly farms out the work of background checks, online material could become a part of every employment dossier. "It's easy, cheap and available," says Donald Harris, head of HR Privacy Solutions, a consulting firm. Recruiter Nick Corcodilos, host of Asktheheadhunter.com, adds that search firms "like to turn up dirt because it proves that they're doing their job."

Even if material is removed, little on the Web ever really disappears. But online search engines like Archive.org's Wayback Machine are actively recording everything that has ever appeared on the Web. Will research firms mine this source? "I'd be surprised if they haven't already," Harris says. "I've used it."

Here's what parents and their children should consider doing:

• HAVE A SIT-DOWN Counsel teens and even postcollege kids to be aware of the potential long-term consequences and to understand that their Web identity need not include everything about them.

• DISGUISE Advise kids to avoid using any info that can be pieced together to fully identify them, including names, phone numbers, addresses and even school names. MySpace itself offers plenty of privacy options that can partly mask your identity; asked for comment, a company rep said the site generally encourages common sense.

• TEAR IT DOWN Once personal info gets disseminated on the Web, it's up to the particular site to decide whether to remove it. But you can always change your MySpace profile. Even if the party animals have left the barn, it's still worth closing the door.