| "Dad, what do you think about heroin?"
| "If you go near that stuff, you'll be grounded for a month."
| "Thanks, Dad." (proceeds to try heroin)
In the time I typed this, probably 1,000 kids around the world had the very same conversation with a parent about various difficult subjects including drug use. That's 1,000 missed opportunities for parents to have real conversations with their children.
I have a few thoughts about the rest of your comment.
First, you're defining "addiction" very broadly. It's hard to reconcile why your girlfriend is a "victim," but "literally every parent" on the playground was doing the same thing. At a certain point, behaviors become norms. If phones didn't exist, what would those parents on the playground be doing instead? Probably not heroin, but probably not work that society highly values, either. And nothing so extreme that it's my business to judge them.
Second, it's very difficult to address your "the question is really about addiction" point without conceding a false premise. My comment wasn't about how I deal with it; it was about how to prepare my children to deal with it. Which absolutely is about addiction, or rather about avoiding addiction. (To be clear, I'm talking about addiction in the classic sense that causes a person to make destructive life choices to feed their addiction, and absolutely not about the "addiction" that declares that there is a problem with a parent's manner of sitting on a bench on a sunny day in a park.)
If OP were asking how to stop mobile-phone usage, I'd have ignored the post entirely, because in my opinion that's a misguided goal. OP asked "do you take any precautions to help your children avoid falling prey to [internet addiction]?" That's a very different question. And my answer was to teach my children not to become prey.