Huge problem in our house even though only DD1, age 15, has a phone. She got it last Christmas, 9th grade so late to the game. She didn't use it a ton at first, but this summer she went to a fantasy summer camp that she adored and a group of them use message boards to communicate year-round, come up with new plot developments, etc. Then a sub-group spun out into Google Hangouts. Since then, I have had far less an idea of who she is talking to, what sites they are referring her to, and her obsession has grown.
I suspect that for many families, a teen or tween's use of devices will change drastically at some point and something that seemed in control will cease to be. That is probably a good argument to start some controls and limits early, and then modify as needed rather than try to develop down the road.
D1 also has a school laptop she brings home daily from school. It's a huge problem. Although it does block access to some websites, there are plenty she can still get lost in like youtube, a fantasy author message board, camp message boards. So she constantly uses her computer for homework (ostensibly) and therefore has simultaneous access to the very things that distract her from doing her homework.
DD2 uses a ton of screens, but pretty benign stuff. Her hours spent watching a screen are high, but I don't have a problem with what she is viewing or doing. She is 13 and does not have a phone.
DS is addicted to youtube and wherever it leads him. He is the youngest (age 9), has the most time, the most interested in chatting on games like Minecraft, the most prone to react emotionally to strangers' comments on the internet and is the most influenced by the junk on the internet. It's a terrible combination. But for whatever reason, I find it the hardest to stop him from using it.
One thing that I notice is that DH has a much higher tolerance for screen time of all kinds. He always uses a screen to relax, to the point that his household chores are very effected. For example, he regularly yells at DS to get off the computer while DH refuses to even take his eyes off the screen he himself is using. He and I are on very different pages about kids and screens, and it's a very hard battle to fight alone. I use my laptop a great deal as well (although my phone much less) so we parents are not the best role models.
Advice and commentary on living overseas
DD1 15, DD2 12, and DS 9