Educate, educate, educate and build trust in your relationship.
Once they have a device it's a snowball you won't be able to stop. So educate them to do what they can to keep themselves safe, to come to you when there is something they don't like and to be mentally strong because whether you like it or not they are going to experience things you would prefer they didn't.
Educate them about screen time and FOMO but don't forget about the good things that can come from it.
We were reasonably strict with our first (now 21). We stopped her from signing up to various channels (or at least tried), insisted we be contacted by schools and coaches etc and it was a massive struggle. Other parents and teachers just didn't care and saw us as freaks for wanting to help her navigate it all.
We realised with our second (now 19) we were powerless to stop him from being signed up to anything and everything by schools, sports clubs and associations without our knowledge or consent so we talked with him about it regularly.
With our third (14) we've completely given up. Everyone of her friends, teachers and coaches use a different method to communicate. We live a fair distance from her college friends and so her being in her room, listening to music, talking to her friends via whatever method she can is important to her. We don't monitor her like the others but we continue to talk about her time on SM, what she's using, what she is seeing and what she's doing herself.
The older two check in regularly too which is nice because although I think I'm pretty clued up on all this in truth I have absolutely no idea what they are signed up to and doing.