It is really, really hard. A vital part of being a teen is "not being left out of the group" so you don't want to make them in to social pariahs, on the other hand, imo, their access to technology must be closely monitored and curtailed. It's getting the balance right that is the difficult thing.
To provide a bit of context: my dd is 12 yrs and has just started secondary school (they start a year later than the UK where we live). She has a fairly intensive school day + ballet classes so we do allow her access to her Ipod + laptop (and occasionally an old mobile phone) during the week with strict rules attached. (She does 4 hrs of sport at school per week plus 4 hrs of ballet per week too so I know she is getting plenty of exercise).
She (and a friend) have already experienced one incidence of mild but nonetheless unpleasant cyber-bullying from girls one class above her (I know these girls individually and their parents and they are generally lovely. Get them together on-line in a group though and the dynamic changes dramatically.)
We have also had one scare with dd when we discovered she was talking to a stranger on line through the Star Stable game (I was popping in and out of the room she was in at the time and thought she was talking to a school friend 
). We were horrified when we discovered the truth as it flouted all of our rules and dd, despite warnings, was completely unaware of the danger she had put herself in. She said "but I do know x because I have been playing SS on-line with her for two years" 
It was all sorted in the end, and she was indeed playing with a girl her own age (dh checked) but talk about scary.
Anyway ... given all the background above
- we have talked again extensively (in an age appropriate manner) to dd about stranger danger, pornography and the dangers of putting too much information about yourself on line, the nasties on Youtube etc. Some other v helpful mothers on Mumsnet pointed me in the direction of the NSPCC and CEOPS websites, both of which contain lots of helpful material on this subject.
- Friday nights aside, on school nights dd is allowed no screen time (including telly) before all homework complete (and completed well and checked by us)
- no more than 1 hr of screen time per school evening allowed (it is usually much less than this owing to hwk/other demands)
- no more than 2 hrs screen time per day allowed at weekends (excluding family telly watching but we don't watch that much)
- no more than 3 hrs screen time per day allowed during school holidays
- she is only allowed to surf the computer downstairs where we can check it from time to time
- we have parental controls on all devices (she mainly uses lap top and Ipod) and occasionally, an old mobile phone
- 30 min gap between end of day and bed-time (no screens)
- no technology of any sort allowed in the bedrooms ever
- only allowed to communicate on line with people she (and we!) know in person - this rule will have to evolve as she gets older obviously
- we have access to her e-mail account and all her passwords and she knows that we regularly check and read her messages both received and sent
- we monitor the music videos she is looking at
- no screens at dining table during family meals, or when (adult) friends and family are here and she needs to join in with the group
Sorry for essay. I sound like a harridan don't I? Sadly, I think these rules are necessary in this day and age. As we found out, it is the wild west out there atm.
Having stated all of the above, dd has just asked me is she can join Instagram "as all her friends are on it". Any advice would be appreciated as I don't really know anything about it!