Sure, there are real dangers associated with all this screen time — everything from cyberbullying to couch-potato obesity. Not to mention driving while texting, shortened attention spans and Internet porn.

But many of today's parents spent hours as kids sitting in front of screens too — only they were TV screens.

Which raises an interesting question: Is Facebook really worse for teenagers' brains than the mindless reruns of "Gilligan's Island" and "The Brady Bunch" that their parents consumed growing up?

But research has shown that the more time kids spend in front of screens — whether it's TV or instant-messaging — the worse their school performance. "That doesn't mean it's true for every kid, but it makes sense, that for every hour a kid is playing video games, it's an hour that they're not doing homework or reading or exploring or creating," said Douglas Gentile, a child psychologist and associate professor at Iowa State University in Ames, Iowa.

Gentile calls this the "displacement hypothesis. If screen time is displacing doing their homework, that's bad. But if their homework is done, well, so what?"

Gentile, who admits that his own teenager crossed the "9,000 texts in one month barrier" last summer, acknowledged that parents are struggling to adjust to a world in which kids would rather look at words on a cell phone screen than have a conversation.

Watching TV as a family, as mindless as that experience can be, is now regarded with nostalgia by parents. If your kid is sitting in the living room watching "American Idol," you can plop on the sofa with them, and "it's a shared experience," Gentile said. But if they're texting or video-chatting with a friend from school, "it's a private experience. It's like they're whispering secrets. And we find it rude."

Patti Rowlson, a mother of two in Everson, Wash., says this "has been a topic of discussion in our house for years now." She and her husband started out limiting TV time when their kids were little, but "then technology crept in. Cell phones, laptop computers, iPods with Wi-Fi. We, as parents, were no longer in control of screen time because we could not even tell when they were using it."

Recounting a struggle that will sound familiar to many parents, Rowlson said that at first, she and her husband imposed limits on tech use.

"There were battles and even groundings," along with the confiscation of iPods, she said. "We were constantly policing and the kids were constantly getting in trouble. We were trying to fight for the old ways, and it was causing a lot of stress and tension in the family. It was ridiculous. So we loosened up. And it's made everybody happier. We were fighting something that you can't hold back. It's how they communicate with their peers."

What's been the result? Two good kids, she said. "In the end I'm not sure if having boundaries early on helped them or made no difference at all."

The explosion in teen screen time is well-documented. A recent Associated Press-mtvU poll found that one-third of college students use computers, cell phones or gaming consoles for six or more hours daily. A Kaiser Family Foundation study published in January found that total media use among 8- to 18-year-olds, including TV, music, computers, video games, print and movies has increased from six hours, 21 minutes daily in 2004 to seven hours, 38 minutes in 2009.

"Try waking a teenager in the morning and the odds are good that you'll find a cell phone tucked under their pillow," the Kaiser report said.

The Kaiser study also found that the more time kids spend with media, the lower their grades and levels of personal contentment are.

Gentile said the impact of screen time on school work can be mitigated by what he calls "protective factors." Those might include good teachers and a high-performing school, love of reading, coming from a family where education is valued, and exposure to experiences that are culturally and intellectually enriching. "If you had all these protective factors," said Gentile, "then that one little risk factor (screen time), who cares?"

He added that surprisingly, the amount of time kids spend watching TV has not declined precipitously with the popularity of computers and gaming, but "they don't pay nearly the attention (to TV) that they used to." The TV might be on, but "they're also instant-messaging, they're on Facebook, they're texting."

One thing parents should worry about, Gentile said, is the way electronic devices encourage multitasking.

"Multitasking is not really good for anyone," he said. "Your reflexes speed up, you're quicker to look over your shoulder and notice little noises or lights. This is not what they need when they get to the classroom and you're supposed to ignore the kid next to you. Scanning to see when the next message comes, this may not be good for kids. The more distractions you have, the worse your performance is." Getting kids to turn off their phones, iPods, and computers in order to concentrate on homework and reading, he said, "I think that's a fight worth having."

Bottom line: Never mind that your kid is spending two hours on Facebook each night. As long as they do their homework without texting in between math problems, it's probably no better or worse than the hours you spent watching "Star Trek."