All DD2(13)'s group use instargram and I confess I haven't a clue when she opened the account.
DD2 stopped asking permission to do computers things when she was about 10.
I'm a bad mummy, I do not stress about it, she knows the rules, she had a lap top at 6, they are tattooed on her soul. No real names, no addresses, no school name. Not that that matters much because their bus goes door to door.
She is a child of the web, she could navigate the BBc website before she could read. For good or ill YouTube, instergram and KIK messenger are her world. She got bored of waiting for DH to set up her email account so she just set up her own google one.
Yes I could pick massive fights and try and watch her every move, but she'd hate me and be a 1000x more likely to do something stupid or not tell me something important.
She knows DH is a professional geek, she knows big brother Linux email server could spy. Her laptop is her pride and joy, she wouldn't risk it.
(Also I assume DD1(16) is/or could become her friend in Instagram, I know she uses it occasionally and she would look out for little sis if necessary.)