When DS was 7yo we figured out that he would literally play computer games 24/7 given a chance, he had no self-limiting mechanism.
So we instituted lots of screen time rules, fairly flexible, needed for sharing time with siblings, too. WiFi now goes off at 8:30pm for instance.
We found that DS didn't overdo it with his DSi so allowed him to get an iPad for birthday last year (13th).
Lately he's using massive amounts of bandwith watching Youtube (currently banned, used 10 GB in 36 hours), tending to not go out or do anything else. I think key solution is to encourage him to have other interests (I have a few ideas) so that the iPad naturally reduces to something reasonable. Think we need more than one strategy, though.
I probably wouldn't mind if it was all FB, but a lot of it is watching videos, shooting games, "free" apps. He can't do much Minecraft on iPad and I wonder if it would be better to give him more Minecraft time on proper PC, but then would have to reconcile with how much screen time other DC get.
What do you all do to keep your teen's screen time at a moderate level?