99% of the problems people experience on social media are due to not having it set up correctly in the first place.
As long as you keep it private, and only add people you know really well, you won't have any problems.
Also, be careful about the games you play and the applications you add through facebook .
When you add something, it will ask you to accept a disclaimer which tells you what information it will access.
Lots of them can access all of your private information , and some of them are quite malicious with it - e.g sending messages to your friends list advertising products without your knowledge.
You can definitely block people and messages from certain people.
You can also choose who sees what you post, each time you post something.
If you don't want the world to see your photos , you can set it so only people you have added as friends can see them.
There are lots of tutorials on the web , which will teach you how to setup privacy , account settings and the likes.
It's basically all about setup, and being smart about what you post / use.