DD has a history of making poor decisions online.
I allowed her to have a very closely monitored Deviant Art account when she was 13 (as art is something she's good at.. my reasoning was she could benefit from a gentle entry into social media while bolstering her confidence by showing off her artwork at the same time). Within days she was getting into arguments and interacting with strangers - one of whom I'm sure was trying to groom her. So the account was shut down.
A few months later, she was desperate to have an Instagram account, like all her friends, so we set one up, with the conditions that she didn't a) talk to strangers, b) lie, c) swear. Within weeks she was doing all of these things. I agreed she could keep it if I had the log in details. I kept her account permanently logged in on my phone, with her knowledge, so I could see exactly what was going on. Her behaviour would improve temporarily, but start to deteriorate again.
During the Easter school holidays, she and a friend went shopping to our nearest large town. They were both behaving quite shiftily when they got home, and a check of Instagram showed me that my DD has developed a crush on a boy from another nearby town, who boasts about being regularly arrested and smoking skunk. They'd set up a Snapchat account on one of the other girls devices, and DD had sent him explicit photographs of herself. This boy is, at this point, a complete stranger. No one had any idea who he is. They'd planned to meet him that day, but luckily he didn't turn up.
Fast forward to 3 weeks ago. DD was at her Dad's for the weekend. I received a call from DD's friends mum, saying she was worried about her safety, as she's told the friend she was off to meet this internet "friend" again. I contacted DD and her Dad, and yes, she'd told a pack of lies and was off to meet him again. When we intercepted her and told her she had to return to her Dad's immediately, she was on the train. She took two hours to get home again, and concocted a cock and bull story about missing her stop / engineering works etc. She claimed she was quite shaken by the whole experience, would never put herself at risk like that again, and that she hadn't actually met up with the boy, but I now know she most definitely had.
Since this happened, Instagram has been suspended, and she's had only limited access to her phone, and supervised internet access at home.
On the recent bank holiday Monday, she was again behaving a bit suspiciously, and cancelled out plans to go shopping together, to meet her female friend. I said they were not allowed to go into the large town 10 miles away, but that they could hang out here or at the girl's house, so they did the latter. To cut a long story short, I worked out that she wasn't with her girl friend, and was indeed with her internet buddy again. I reported her to the police, as she was uncontactable, and they found her 6 hours later, in another town, smoking in a park with a bunch of kids who are well known to the police for fighting / drink and drugs. The only piece of good news is that this boy is the same age as her, and is broadly who he says he is, rather than an older predator.
The police brought her home and had stern words (they were brilliant, actually), and since then, DD has been told that to get any freedoms back, she needs to earn our trust by demonstrating a more mature, truthful, and responsible attitude. She needs to put in some effort at school (year 10), and communicate with me, her dad, and her respective SD & SM.
In the meantime, I've found her smoking out of her bedroom window several times. So she's in trouble about that too.
So, she went off to her Dad's again this weekend, and was trusted with her phone for safety on the journey, and lots of communication all round about which trains she was getting, and when she would arrive. A perfect opportunity for her to earn a little trust back from us, but (predictably, I suppose), she went AWOL again, to meet this boy. Her phone had run out of battery, and she borrowed a phone from another passenger on the train to call the police, asking them to let me know her train had been delayed and she was OK! In her mind, this is her demonstrating a responsible attitude, completely ignoring the fact that she'd arranged to meet him in the first place (using Snapchat on her Dad's phone apparently... yes, words have been had with him too) and was still lying to us all.
So now, she's in a situation where she has no phone, no internet access, and can no longer travel too and from her Dad's independently. I've threatened to walk her to and from school too, but am holding off on that as I desperately want her to have at some opportunities to be with her friends and not completely take away every shred of independence.
I found a "to do" note in her blazer this morning, saying "1. re-activate Snapchat, 2. ask someone if I can use it, 3. get a phone, and 4. Find number for suicide prevention helpline". Obviously, this is the most worrying thing of all.
She has had some intervention from CAMHS in the past, but has nothing but contempt for them and refuses to talk to the GP about how she's feeling. She said she will talk to the school counsellor, but I'm not convinced she will, nor am I convinced of the counsellor's effectiveness anyway.
I think I might take her to the GP after school anyway, as if she's having suicidal thoughts, that's a medical / MH emergency - right?
But... what now? I can't keep her wrapped up in cotton wool, but I can't allow her to hang out with this drug taking fighter either. She is very vulnerable... she has dyspraxia, is very easily led, and is like a much younger child in so many ways. She doesn't seem able to think about the consequences of her actions, and says she doesn't care about her future. He best friend has told me she wants to have sex with this boy, and I know from her texts / social media in the past that she's very interested in trying skunk.
Just really hoping someone has some experience of turning a situation like this around 
I'm sorry this is so long. Thank you for reading it.