My 14-year-old just joined Facebook and I’m nervous about who they’re talking to and what they’re posting. How do you keep an eye on things without totally invading privacy?
Totally get the nervousness - Facebook’s a wild place for a 14-year-old. Honestly the easiest first move is just making sure you’re Facebook friends with them and that their profile is set to “Friends only” - takes 5 minutes and covers a lot of ground.
Facebook also has Messenger Kids controls but since they’re on regular Facebook, check out Meta’s Family Center (families.facebook.com) - it’s their official parental supervision tool that lets you see who they’re talking to and set time limits without full-on snooping. 
Long-term though, having a casual “show me your feed sometimes” relationship with your kid beats any app - they’re more likely to come to you if something weird happens.