As a mom or dad, you hear it all the time. Too often. It’s one of those firm parenting axioms recited by smug sages — like “sleep when your baby sleeps” — that’s as nonnegotiable as it is unachievable.

“Children don’t need a friend,” the advice goes. “They need a parent.” And it’s true. Except on Facebook, where it turns out to be entirely false.

After years of careful evasion, my husband and I finally let our 8th grader create a Facebook account. We’d been holding out, we said, because publishing personal information to hundreds of people requires a modicum of maturity; crude comments and damning photos can have disastrous consequences.

But here was the real reason: We didn’t want him to see our crude comments and damning photos on Facebook: The status updates whining about our kids’ whining. The picture of dual-mounted street signs at the intersection of Inyo and Butte. The absurd pages I support, including one called “When I was a kid I thought Cal Worthington said ‘Pussycow,’ not ‘Go See Cal.'”

But our reasons for keeping the kid off social-networking sites (“Beware the cyber bullies, whatever those are”) were growing thinner, and our hypocrisy (“We’ll discuss this later, son; I’m busy on Facebook now”) ever fatter. So we caved.

We set up basic parameters for privacy (only “friend” folks you’ve met in person), courtesy (accept any friend requests from family members), and personal responsibility (schoolwork comes first). We warned him that all activity can potentially be seen by future employers, college admission reps, and girls who will never, ever date him because he listed The Zombie Survival Guide among his favorite books.

We took some friends’ advice, too: We bid adieu to our own Facebook freedom (farewell, fair crudeness!) and friended him so that we could monitor his netiquette and make sure he’s using the tool conscientiously — the way we’d stand guard if he were using a Cuisinart or a nail gun.

Case in point: A friend of mine asked her high-schooler to unfriend his Grammy because of “too-frequent F-bombs.” To clarify, his F-bombs, not Grammy’s.

Being friends with your kids on Facebook provides invaluable, and otherwise invisible, insight to their social worlds. But you’re supposed to lurk surreptitiously rather than posting “Love you, sweetie!” on their public pages — as my son’s auntie did, to his utter astonishment and horror. In other words: You’re supposed to be a friend, not a parent.

“I wrote on my daughter’s wall once and got reprimanded big-time,” one mom told me. “She said it was like I ‘kissed her’ in public and a total embarrassment.”

The real Facebook friend-emonium comes at the next level, though: Do you friend your kids’ friends? Much as I truly — truly — don’t want to know when my son’s pals change their relationship statuses to “it’s complicated” (Please. Really?), it’s nice to know grown-up eyes glimpse every word they type. It’s the digital equivalent of a small town: No underage beer reference or oh-no-she-didn’t bra snapshot goes unnoticed.

“I’m friends with all my closest friends’ parents and my mom,” said a junior-high girl I know. “I don’t have a problem with it until I’m up really late with my friends writing stuff on Facebook and their parents comment, telling us to go to bed.”

“My son’s friends friend me all the time,” says the F-bomber’s mom. “I like it, but it drives him crazy — especially when I hear hot gossip before he does.”

Wait, now, we can’t be parents, and we can’t be friends, either? And we can’t even whine about it on Facebook?

Status update: Help. Where are those smug sages when you need ’em?