I have a teenage daughter who doesn't do anything except use her mobile phone for social media (Time Limits) given up on her hobbies, isn't interested in achieving in school, although she could and is only interested in boys.
sounds exactly like my 13 year old she does nothing except hang around outside kfc, parks and other places like that. when at home she is glued to her phone. I drag her out to visit family occasionally and make her walk the dog etc. I just don't know what you can do, other than make sure they tell you where they are/who with, and impose curfews etc. I'm hoping when it gets nearer GCSE time they will have something to focus on that will keep them busy. Good luck it's grim i know!