Facebook is a lot of fun. It's a place for friends, and friends of friends. I have a lot of friends, some of whom I have met IRL (in real life) and some who I will never meet that way. My friends are people that I know here in Arizona, or knew in California or Minnesota. They're people that live all over the world, and share common interests with me, such as a fascination for cycads.
When you post something, you have the option of selecting public, friends, or only yourself. Personally, I post everything to public, as I want everyone in the whole world to know that I like dachshunds. Many people prefer to post only to friends, and although I'm not sure what posting to yourself is for, I'm sure many people find that handy.
This, however, only controls what posts people see. That is, when you write something, such as "I like dachshunds", or post a photo. It doesn't control comments.
Comments are not the same as posts. Comments are the words that you write after a post has been made. It could be "I like dachshunds, too!" (and I hope it would be). It could be something horrible and nasty (and I hope it won't be).
I hope this helps. Yes, there are a lot of privacy settings for social media, but people are coming to the realization that if they say nasty, venomous things, it can get back to them. So my recommendation, not just as a web guy, but as a fellow-traveler of yours on planet earth, is to be true to yourself, consider the feelings of others, and share the world.