Social media can't be monitored, it's all or nothing and I don't know what to do about my teenagers

Both of my children have had tablets and phones completely unmonitored since they were 7 years old. Once they turned 13, I allowed them to join social media sites like Facebook and Instagram. I warned them of all the dangers of sharing personal info, online predators, bullying, everything. I always trusted them completely. That was a huge mistake.My daughter tried to kill herself over something a "boy" said online, which was probably an adult man masquerading as a young, cool dude. My daughter considered him to be a "boyfriend" even though they've never met and she really knows nothing genuine about this person.I forced her to show me her phone and I found some seriously raunchy sexting messages and she has given away far too much personal info.I looked into Instagram and it is incredibly easy to hide/erase your tracks. If you delete a message, it deletes it from the other person's phone as well. So, for example, I tell my daughter to block the guy she is talking to. She does so and I make a note of the last message she sent him. Then when I'm not looking, she can unblock him, they can chat up a storm, and then just delete the messages that were sent since the last time I checked her phone and they'd pull a fast one on me.It seems like the only possible way I could trust her again would be to install a keylogger on her phone and check it daily, then ask her to see if she lies about what she did on the phone that day.I think social media can be a good and healthy thing (memes are hilarious and awesome) but it seems like there's no possible way to allow her access to some but not all of it. It's all or nothing and I hate that resolution.Any experiences or thoughts you can share? via /r/Parenting https://ift.tt/2BZSPE9