My teenage daughter — we’ll call her Franny– caught a friend in a lie tonight.

The kind of lie where the friend was actually blowing Franny off to spend time with someone else, but couldn’t just say that (the way my daughter would have), and instead gave some lame excuse that she couldn’t because, um, “my Mom is being weird.”

Frankie found out because, of course, a different friend posted on MySpace that she was waiting for this same person to come over. So, duh, this girl is not grounded because her Mom was being weird.

Now, this post isn’t about how easy it is to catch people in a lie, thanks to social media, though that’s still true. (My favorite one of those stories is still the Bank Intern and the Fairy Wand).

Nope. Apart from the easy slander on Moms — crazypersonblamecatchers that we are — which I’ll ignore for the moment — this is really about the lifelong process of growing and culling your friendships, and finding those people who are strong enough to tell you the truth. The big huge truths, and the little stupid ones (“Uh, Dude? Is that the same shirt that Chris Farley wore in Tommy Boy?”). And you know what, Franny’s in high school — where people are organized by AGE, for heaven’s sake — so it’s hard for her to understand that there is a world out there where she’ll be able to find and recognize friends because they have things in common beyond the year they were born and whether they’re willing to have lunch together.

Mind you, Franny’s hurt and furious. “How could she lie like that?,” she asks, rhetorically. “How could she not think I’d find out?”

See, at Franny’s age, and the age in which she’s living, friends are literally and figuratively a currency. How many do you have?

But soon, I hope — and I hope that social media stretches to help — her understandings of friends will deepen. It’s not just about who will go to a party with you, or who shows up with a bright smile, posed in your Photobucket.

It’s about finding the people who will appreciate and forgive you when you’re not perfect. Who will be straight up when they have to disappoint you, knowing they’ll be forgiven — but in a way that meets you as an equal, and takes responsibility for errors, not that careens into you and leaves the scene of the accident.

Franny is stronger than she knows, and she’s going to be “one hell of a woman,” as the saying goes — because she’s so, well — frank. It may cost her some friends, and her lesson may be to occasionally learn when to turn up the tact and turn down the honesty.

Tonight, Franny wanted me to tell her she was right, that this girl was terrible– that she should just cross her off her list of friends.

But I wasn’t going to. There’s a place for that kind of friend — you don’t make that kind of person your best friend, because she’ll never be strong enough. She is what she is, and you know it. Franny should sigh, shake her head; maybe say, “that was stupid,” and let it go. And then go on, looking for the right kind of friend — which is, actually, just as hard to find sometimes as a lover. Someone who is fiercely honest, funny, smart, opinionated — and who, just like Franny, will occasionally need to be forgiven.