First, it's about trust. Trust starts when they're infants. They cry, you show up. They cry, you show up. They have to know that when they have a need, you provide for it. Trust is also about leading by example. Be the person you want your children to be. If you don't want them on Facebook, don't be on Facebook. If you don't want them to use a smartphone at the table, don't bring yours to the table. Will this, in reality, mean they'll not use Facebook or bring their phones to the table? Maybe, maybe not. You don't want robots. You want independent humans. You're a guide, an example, that's all. Be the best one you can be. But when they do bring their phone to the table, you will have, from birth, built up enough trust to be able to have a conversation about it with them. They'll talk to you because they trust you. When you have a concern that you think they're forming an addiction to their phone, they'll talk to you, because you've earned their trust.
What if they don't? I've met a few parents who thought they had parenting nailed. Then they had another child.