I can't realistically monitor what my children are looking 24/7, like I can't listen to every conversation that they have.
I could ban all social media / phones but how is that going to prepare them for life in a world where social media is all encompassing? Plus they likely would be ostracized by their peers.
[I'm talking here about older children, teenagers. We also have a 5 year old and it's pretty easy with him, he doesn't have access to any form of social media.]