I had monitoring software on my kids computers and had their social media passwords, but you're right, once they got to be older teens, I didn't go digging behind their backs. But they knew I could. My daughter has special needs, and even though she's 25, I still have access to her accounts, but I wouldn't go checking them without discussing it with her, and I would have her present when I did.A 16 year old deserves a right to privacy, they are still vulnerable but they are young adults and shouldn't have parents putting monitoring devices on their phone, hawk eyeing all their social media, checking their browser history or going through their text messages, what a massive way to break trust and control them. Safe guarding for 16/17 year olds means talking, letting them know how to use the internet safely, discussing the pros and cons of what can happen and treating them like the young adults they are. If the person is a child under 16 then it's a different story but I'd be mortified at that age knowing that my parents are watching my every move.
You're right that there has to be some trust, and kids have to have some privacy, but there's a line to walk between respecting their privacy and being oblivious, which it seems so many parents are. Groomers couldn't succeed at online grooming if parents talked to their kids, and kept an eye on their accounts. That's why they say that kids shouldn't have computer in their rooms. If it's in a family area, there's less likely to be an opportunity for someone to creep on them.