The New York Times Style Section has a classic essay (reg. required) from a mom of a teen who joined Facebook. She writes:

Things were going really well, when suddenly something disturbing happened. An instant-message window appeared onscreen to deliver a verdict.

"wayyy creepy," it said. "why did you make one!"

Ah, there she was.

"What are you talking about?" I typed innocently.

"im only telling you for your own good," my daughter typed.

"Be my friend," I typed.

"You won''t get away with this," she typed. "everyone in the whole world thinks its super creepy when adults have facebooks."

"Have facebooks? Is that what you think a Profile page is called?" I typed.

She disconnected.

First of all, I love this mom. She took the bull by the horns and created her own profile, invited some friends and figured out why social networking can be fun -- especially for teens. The question she asks later in the article is also poignant:

If the presence of people like me alienated Facebook's core younger group, would they flee? And if so, whom would I annoy?

The writer perfectly captures that age old mother/daughter tension. You can almost feel her daughter's anxiety and see her eyes rolling as she types back and forth with mom. I definitely think teens are on these sites to socialize with each other -- just as they would in person, in their bedroom, with the door closed and the "keep out" sign posted. It's a normal part of individuating from parents and figuring out who they are. But just as you might knock on their bedroom door carrying a track of snacks, and check in on your teens, I think it's ok to do this on social networking sites. The key is finding the balance between letting them have their space and staying involved and engaged.

I actually don't think teens will leave Facebook if mom or dad signs up. They may up their privacy settings so most of their "Facebook" is only visible to their friends. When Facebook opened up to teens, college students were upset, but they're still there. Now that it's open to everyone, I just think people will use privacy settings to better establish exactly who they want to be networking with.