Let your kids know that you are available to talk with them and give them advice. Make sure they know they can talk to you and feel comfortable doing it.
Tell your child to turn location services off on their posted pictures. If you constantly post about where you are, where you are going, etc., you could be a target out in public, or someone could come to your house. Inform your child of what could happen in these situations.
If you feel that it is necessary, keep your computer in a central location where you are able to monitor what your child is doing.
Allow your kids more access as they get older so they can see that you trust them with their abilities to make good choices. It allows them to implement what you have taught them and go out into the world knowing what to do.
Here are some statistics about internet safety on such topics as cyberbullying
and overall statistics
And, here are a couple of videos about safety online that you may want to show your kids.