In case you have not noticed, Facebook has a real overshare issue, in that it defaults to the assumption you want the whole world to know all of your business. A friend and I talked about it this afternoon, and a couple of facts came out of our conversation that other people might not have thought about.
1. When you comment on a person’s posts, everyone you are connected to who is able to see that post will get a notice in their news feed showing the post and all of its comments. The only exception is the person who has gone to your timeline and turned off the option to get those notices in their own news feed. They will still be able to see the post and your comment, if they go to that poster’s timeline; they just won’t get a post in their news feed essentially saying, "Look at this post!!!!!!!! ——>>>>>>> YOU KNOW YOU WANT TO READ IT!!!!!"
2. Always check a post’s visibility before commenting on it. If the post is "Public", the whole internet will be able to read your comments on that post.
3. A year or so ago FB gave us the ability to change the visibility of our posts after the fact. It is a useful feature for those of us who sometimes want to filter a post we have already made. But the filtering can work the other way around, too. If you comment on a post that is locked in some way (e.g. filtered or friends-only), the post’s owner can, at any time, make that post public, at which time your comments will be visible to the entire internet.
In short, be mindful of what you post on Facebook, not just on your own timeline, but also in comments to other people’s posts, close friends and family included. What you intended for only a few eyes could, at any time, be made visible to the entire world.